xen, libxc: rename xenctl_cpumap to xenctl_bitmap

xen, libxc: rename xenctl_cpumap to xenctl_bitmap

More specifically:
 1. replaces xenctl_cpumap with xenctl_bitmap
 2. provides bitmap_to_xenctl_bitmap and the reverse;
 3. re-implement cpumask_to_xenctl_bitmap with
    bitmap_to_xenctl_bitmap and the reverse;

Other than #3, no functional changes. Interface only slightly
afected.

This is in preparation of introducing NUMA node-affinity maps.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>
Acked-by: Keir Fraser <keir@xen.org>