core: return QubesVMNotFoundError when setting non-existent VM to VMProperty
core: return QubesVMNotFoundError when setting non-existent VM to VMProperty

Not sure about this one. Maybe it should be rather
QubesPropertyValueError?

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