libxl: an 'unknown' shutdown reason may manifest 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>
Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
libxl: ocaml: fix bitmap alloc in xenlight bindings

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>