libxc/xentrace: Use xc_cpumap_t for xc_tbuf_set_cpu_mask

libxc/xentrace: Use xc_cpumap_t for xc_tbuf_set_cpu_mask

xentrace is the only caller at the moment.  Split the cpu and event
mask setting out into seperate functions, but leave the current limit
of 32 bits for masks passed in from the command-line.

Based on a patch from Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>