dispvm: fix firewall propagation when the calling VM has no rules set
dispvm: fix firewall propagation when the calling VM has no rules set

Fixes QubesOS/qubes-issues#1608

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