udevadm_info() fails if any value contains "="

Description:
  curtin.udev.udevadm_info() uses split("=", 2), but intended to use split("=", 1).

  This was observed while attempting to install Ubuntu 20.04 on Nutanix AHV.

Example:
  ubuntu-server@ubuntu-server:~$ sudo udevadm info --export --query=property /dev/sda | grep '=.*='
  SCSI_IDENT_TARGET_VENDOR='clusterid=58296284039924012'
  ubuntu-server@ubuntu-server:~$ export PYTHONPATH=/snap/subiquity/1966/lib/python3.6/site-packages
  ubuntu-server@ubuntu-server:~$ /snap/subiquity/1966/usr/bin/python3
  Python 3.6.9 (default, Jul 17 2020, 12:50:27)
  [GCC 8.4.0] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> from curtin.udev import udevadm_info
  >>> udevadm_info("/dev/sda")
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/snap/subiquity/1966/lib/python3.6/site-packages/curtin/udev.py", line 95, in udevadm_info
      key, value = line.split('=', 2)
  ValueError: too many values to unpack (expected 2)