Incorrect handling of virsh `domblkinfo` output

(originally found by ~peppepetra)

In some cases (network storage devices?), virsh `domblkinfo` command will return the following output:

```
Capacity: -
Allocation: -
Physical: -
```

(reference: https://github.com/libvirt/libvirt/blob/d725932a283fdc8b827d83cfc5d54b38cf40592b/tools/virsh-domain-monitor.c#L419)

This will cause rack to fail the virsh discovery with the following traceback:

```
2023-06-27 13:30:41 provisioningserver.rpc.pods: [critical] Failed to discover VM host.
        Traceback (most recent call last):
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in errback
            self._startRunCallbacks(fail)
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 568, in _startRunCallbacks
            self._runCallbacks()
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 654, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1475, in gotResult
            _inlineCallbacks(r, g, status)
        --- <exception caught here> ---
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/snap/maas/28584/lib/python3.8/site-packages/provisioningserver/drivers/pod/virsh.py", line 1411, in discover
            discovered_machine = yield deferToThread(
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/snap/maas/28584/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/snap/maas/28584/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 202, in wrapper
            result = func(*args, **kwargs)
          File "/snap/maas/28584/lib/python3.8/site-packages/provisioningserver/drivers/pod/virsh.py", line 713, in get_discovered_machine
            size = self.get_machine_local_storage(machine, device)
          File "/snap/maas/28584/lib/python3.8/site-packages/provisioningserver/drivers/pod/virsh.py", line 590, in get_machine_local_storage
            return int(self.get_key_value(output, "Capacity"))
        builtins.ValueError: invalid literal for int() with base 10: '-'
```

The workaround is to get rid of such storage devices and perform the discovery again.

I think we might want to handle '-' response separately. For a proper fix, we need to clarify in which cases virsh returns such responses and figure out how exactly do we want to handle storage in those cases.