guest_config_minidisks would conflict when called more than one time

guest_config_minidisks is to update a minidisk, so the vdev should be required to specify which disk to update.

Current code would each time use root_vdev+1 as the first disk and /mnt/ephemaral0 as the first mount point. So when the guest_config_minidisks is called for more than one time, unless user specify the vdev and mnt point explicitly, would lead to conflict.

    def process_additional_minidisks(self, userid, disk_info):
        '''Generate and punch the scripts used to process additional disk into
        target vm's reader.
        '''
        for idx, disk in enumerate(disk_info):
            vdev = disk.get('vdev') or self.generate_disk_vdev(
                                                    offset = (idx + 1))
            fmt = disk.get('format')
            mount_dir = disk.get('mntdir') or ''.join(['/mnt/ephemeral',
                                                       str(idx)])

From users' perspective, I think this is unreasonable.