qemu: fail on attempts to use <filterref> for non-tap network connections

qemu: fail on attempts to use <filterref> for non-tap network connections

nwfilter uses iptables and ebtables, which only work properly on
tap-based network connections (*not* on macvtap, for example), but we
just ignore any <filterref> elements for other types of networks,
potentially giving users a false sense of security.

This patch checks the network type and fails/logs an error if any
domain <interface> has a <filterref> when the connection isn't using a
tap device.

This resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=1180011