monitor: Add -a (all) option to info registers

The info registers command in the qemu monitor is used to dump register
values.

Currently this command uses the monitor cpu (which can be set by the
user) as the cpu for whose registers will be dumped. Sometimes it is
useful to see the registers for all cpus and currently this requires
setting the monitor cpu and the re-running the command for each cpu
in the system. I would be nice if there was an easier way to do this.

Add the "-a" option to the info registers command to dump the register
values for all cpus.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>

Message-Id: <20170608054116.17203-1-sjitindarsingh@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>