libxl: remove obsolete check for result of xc_get_max_cpus
libxl: remove obsolete check for result of xc_get_max_cpus

xc_get_max_cpus from Xen version 4.3 may return 0 in case xc_physinfo
fails. This has been fixed in Xen 4.4. Remove the obsolete result check
from libvirt. Just convert libxl error codes to plain -1.

Signed-off-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>
Reviewed-by: Jim Fehlig <39929b5b86d72d51fceac59b52303e3fe8a3083e@suse.com>

zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt,olafhering/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt