Fix logic in LibvirtConnTestCase._check_xml_and_uri

The _check_xml_and_uri method has two loops making it
iterate over different hypervisor types. By default it
will iterate over qemu, kvm, xen and lxc, but it can be
made to only check xen by setting the 'xen_only' param
or the 'expect_xen_hvm' parameters.

Unfortunately the second loop is only filtered if the
'expect_xen_hvm' parameter is set, not if only the
'xen_only' parameter is set. This flaw hasn't showed
up before, due to a flaw elsewhere in the libvirt
driver code, cancelling it out.

Change-Id: If70faa05975b726067ab6edaa505ca8d0c230a38
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Fix logic in LibvirtConnTestCase._check_xml_and_uri"