https://bugzilla.redhat.com/show_bug.cgi?id=1515533

The @prefix attribute to <ip/> element for interface type user is
optional. Therefore, if left out it has value of zero in which
case we should not check whether it falls into <4, 27> range.
Otherwise we fail parsing domain XML for no good reason.

Broken by commit b62b8090.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>