sysfs.GetSystemUUID may not report properly (if ever) the VM UUID in Xen guests

[TSnake41]

Xen PV/PVH guests don't have a SMBIOS table, causing the current logic to fail. Aside these guest types (in case we have a SMBIOS table), there are some quirks which may cause it to be wrong (see hvmloader: Update to SMBIOS 2.6 which attempts to fix it).
Linux provides a canonical way of getting the VM UUID under Xen (/sys/hypervisor/uuid), which works regardless of SMBIOS existence and avoid some quirks.

It is documented as stable Linux ABI : https://github.com/torvalds/linux/blob/master/Documentation/ABI/stable/sysfs-hypervisor-xen#L91-L96

What:		/sys/hypervisor/uuid
Date:		March 2009
KernelVersion:	2.6.30
Contact:	xen-devel@lists.xenproject.org
Description:	If running under Xen:
		UUID of the guest as known to the Xen hypervisor.

[dims]

@TSnake41 will you be filing a PR? (please!)