dom0/core: fix QubesVM.gateway
dom0/core: fix QubesVM.gateway

To be consistent with QubesNetVM, where gateway property contain gateway IP for
_other_ VMs, in non-network-provider VM it should be empty.

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