core2migration: try to set properties to "default" when possible
core2migration: try to set properties to "default" when possible

Core3 keep information whether property have default value for all the
properties (not only few like netvm or kernel). Try to use this feature
as much as possible.

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