An 'unknown' shutdown reason manifests as 255, not -1

This should probably be fixed in libxl, consider this a workaround.

Although xen uses -1 as an 'unknown' shutdown reason, when this
is marshalled into a getdomaininfo domctl, it is masked with 0xff
and treated as an unsigned value.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>