qubes/firewall: allow listing only IPv4/IPv6 rules
qubes/firewall: allow listing only IPv4/IPv6 rules

This will allow setting only IPv4-related rules to IPv4 address, and the
same for IPv6

QubesOS/qubes-issues#1815

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