libxl: allow to allocate cpumap with specific size

libxl: allow to allocate cpumap with specific size

Currently, libxl_cpumap_alloc()allocate the cpumap with size of number
of physical cpus. In some place, we may want to allocate specific size
of cpumap.  This patch allow to pass a argument to specific the size
that you want to allocate. If pass 0, it means the size is equal to
number of physical cpus.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>