monitor: Use qemu_get_cpu() in monitor_set_cpu()

No functional change, just a reduction of CPU loops.

The mon_cpu field is left untouched for now since changing that requires
a number of larger prerequisites, including cpu_synchronize_state() and
mon_get_cpu().

Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>