docs: Xen improvements

docs: Xen improvements

In formatdomain, using 'libxl' and 'xen' is redundant since they now
both refer to the same driver. 'xen' predates 'libxl' and unambiguously
identifies the Xen hypervisor, so drop the use of 'libxl'.

In aclpolkit, the connection URI was erroneously identified as 'libxl'
and the name 'xenlight'. Change the URI to 'xen' and driver name to 'Xen'.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>