libxl: fork: Break out checked_waitpid

libxl: fork: Break out checked_waitpid

This is a simple error-handling wrapper for waitpid.  We're going to
want to call waitpid somewhere else and this avoids some of the
duplication.

No functional change in this patch.  (Technically, we used to check
chldmode_ours again in the EINTR case, and don't now, but that can't
have changed because we continuously hold the libxl ctx lock.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>