ocaml/xenctrl: Check return values from hypercalls

ocaml/xenctrl: Check return values from hypercalls

rather than blindly continuing and possibly using negative values.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Dave Scott <dave.scott@eu.citrix.com>
Acked-by: David Scott <dave.scott@citrix.com>
(cherry picked from commit 3380f5b6270e6fa4b24313f8808e7625e4c5a6ba)
(cherry picked from commit c91ed889ea3c3781a94a30909f30c3aad56c97d5)