dom0/core: major rework of QubesVmCollection class
dom0/core: major rework of QubesVmCollection class

No longer individual QubesVm attributes hardcoded in QubesVmCollection
(qubes.xml load). Now it is integrated to QubesVm attributes machinery. Also
QubesVmCollection have no longer hardcoded supported VM types - this will
greatly improve code extension possibilities.

This commit doesn't cover QubesVmCollection.add_*vm methods (which are broken
for now because of missing "collection" argument to QubesVm constructor). This
will be done in next commit.

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