qemuDomainDeviceDefValidateNetwork: Check for range only if IP prefix set

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>

src/qemu/qemu_domain.c

@@ -4373,7 +4373,8 @@ qemuDomainDeviceDefValidateNetwork(const virDomainNetDef *net)
                 }
                 hasIPv4 = true;
 
-                if (ip->prefix < 4 || ip->prefix > 27) {
+                if (ip->prefix > 0 &&
+                    (ip->prefix < 4 || ip->prefix > 27)) {
                     virReportError(VIR_ERR_XML_ERROR, "%s",
                                    _("invalid prefix, must be in range of 4-27"));
                     return -1;