Fix probing of legacy Xen driver to not leave URI set

Fix probing of legacy Xen driver to not leave URI set

When the legacy Xen driver probes with a NULL URI, and
finds itself running on Xen, it will set conn->uri. A
little bit later though it checks to see if libxl support
exists, and if so declines the driver. This leaves the
conn->uri set to 'xen:///', so if libxl also declines
it, it prevents probing of the QEMU driver.

Once a driver has set the conn->uri, it must *never*
decline an open request. So we must move the libxl
check earlier

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>