network: Let domains be restricted to local DNS

network: Let domains be restricted to local DNS

This adds a new "localOnly" attribute on the domain element of the
network xml.  With this set to "yes", DNS requests under that domain
will only be resolved by libvirt's dnsmasq, never forwarded upstream.

This was how it worked before commit f69a6b987d616, and I found that
functionality useful.  For example, I have my host's NetworkManager
dnsmasq configured to forward that domain to libvirt's dnsmasq, so I can
easily resolve guest names from outside.  But if libvirt's dnsmasq
doesn't know a name and forwards it to the host, I'd get an endless
forwarding loop.  Now I can set localOnly="yes" to prevent the loop.

Signed-off-by: Josh Stone <jistone@redhat.com>