libxl: libxl_get_max_cpus returning a libxl error from 4.4 onward

libxl: libxl_get_max_cpus returning a libxl error from 4.4 onward

Starting from commit 2e82c18c in Xen (will be included in Xen 4.4)
both libxl_get_max_cpus() and libxl_get_max_nodes() start returning
a proper libxl error code, in case of failure. This patch fixes
this in the libxl driver.

Note that, although it is now basically impossible for them to return
0, that would, theoretically, still be wrong. Also, checking that the
returned value is '<= 0' makes the code correct for both Xen 4.4 and
Xen 4.3 (and 4.2), and that is why we go for it (rather than
just '< 0').

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>