Use '<option name="option_name">option_value</option>' instead of
'<options option_name="option_value"/>'. It's more consistent with the
rest of qubes.xml - have one thing per element.
Also, add options deserialization test.
Allow device plugin to list attached and available devices. Enforce
at API level every device being exposed by some domain.
This commit only changes devices API, but not update existing users
(pci) yet.
QubesOS/qubes-issues#2257
