libvirt xml: fix DNS address broadcasted by DHCP server
libvirt xml: fix DNS address broadcasted by DHCP server

In R4.0 it is no longer the same as the gateway.

QubesOS/qubes-issues#3585

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