qubes: fix None serialisation for VMProperty
qubes: fix None serialisation for VMProperty

Note that None != default (no attribute in XML at all).

This is reworked
  27eddcb WIP core: fix deserializing None for VMProperty
by Marek.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin