CA-118195: xenlight: set O_CLOEXEC when opening a file for libxl

This change was lost in the recent suspend image conversion work.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #132 from djs55/cloexec

CA-118195: xenlight: set O_CLOEXEC when opening a file for libxl