xen/sysfs: Use XENVER_guest_handle to query UUID

This hypercall has been present since Xen 3.1, and is the preferred
method for a domain to obtain its UUID. Fall back to the xenstore method
if using an older version of Xen (which returns -ENOSYS).

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>