qemu: Implement usernet address

qemu: Implement usernet address

https://bugzilla.redhat.com/show_bug.cgi?id=1075520

Apart from generic checks, we need to constrain netmask/prefix
length a bit. Thing is, with current implementation QEMU needs to
be able to 'assign' some IP addresses to the virtual network. For
instance, the default gateway is at x.x.x.2, dns is at x.x.x.3,
the default DHCP range is x.x.x.15-x.x.x.30. Since we don't
expose these settings yet, it's safer to require shorter prefix
to have room for the defaults.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: laine@laine.org