libxl: improve subprocess handling

libxl: improve subprocess handling

If available, let libxl handle reaping any children it creates by
specifying libxl_sigchld_owner_libxl_always_selective_reap.  This
feature was added to improve subprocess handling in libxl when used
in an application that does not install a SIGCHLD handler like
libvirt

http://lists.xen.org/archives/html/xen-devel/2014-01/msg01555.html

Prior to this patch, it is possible to hit asserts in libxl when
reaping subprocesses, particularly during simultaneous operations
on multiple domains.  With this patch, and the corresponding changes
to libxl, I no longer see the asserts.  Note that the libxl changes
will be included in Xen 4.4.0.  Previous Xen versions will be
susceptible to hitting the asserts even with this patch applied to
the libvirt libxl driver.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>