monitor: Simplify do_inject_mce() with qemu_get_cpu()

Avoids an open-coded CPU loop.

Reviewed-by: liguang <lig.fnst@cn.fujitsu.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
