libxl: only issue cpu-add call to QEMU for not present CPU

libxl: only issue cpu-add call to QEMU for not present CPU

Calculate the final bitmap for CPUs to add to avoid having annoying
error messages complaining those CPUs are already present. Example
message is like (wrapped):

libxl: error: libxl_qmp.c:287:qmp_handle_error_response: received an
error message from QMP server: Unable to add CPU: 0, it already exists

We can also properly handle error from QMP now.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>