libxc/libxl: sanitize error handling in *_get_max_{cpus, nodes}

libxc/libxl: sanitize error handling in *_get_max_{cpus, nodes}

In libxc, make xc_get_max_{cpus,node}() always return either a
positive number or -1, and change all the callers to deal with
that.

In libxl, make libxl_get_max_{cpus,nodes}() always return either a
positive number or a libxl error code. Thanks to that, it is also
possible to fix loggig for libxl_{cpu,node}_bitmap_alloc(), which
now happens inside the functions themselves, more accurately
reporting what happened.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>