monitor: Support specified vCPU registers

monitor: Support specified vCPU registers

Originally we have to get all the vCPU registers and parse the
specified one. To improve the performance of this usage, allow user
specified vCPU id to query registers.

Run a VM with 16 vCPU, use bcc tool to track the latency of
'hmp_info_registers':
'info registers -a' uses about 3ms;
'info registers 12' uses about 150us.

Cc: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20220802073720.1236988-2-pizhenwei@bytedance.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>