vm/qubesvm: write QubesDB /qubes-netvm-gateway6 entry when set
vm/qubesvm: write QubesDB /qubes-netvm-gateway6 entry when set

This is needed for network-providing VM to actually provide IPv6
connection too.

QubesOS/qubes-issues#718

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