libxl: In libxl_set_vcpuonline check for maximum number of VCPUs against the cpumap.

libxl: In libxl_set_vcpuonline check for maximum number of VCPUs against the cpumap.

There is no sense in trying to online (or offline) CPUs when the size of
cpumap is greater than the maximum number of VCPUs the guest can go to.

As such fail the operation if the count of CPUs to online is greater
than what the guest started with. For the offline case we do not
check (as the bits are unset in the cpumap) and let it go through.

We coalesce some of the underlying libxl_set_vcpuonline code
together which was duplicated in QMP and XenStore codepaths.

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