hmp: show all of snapshot info on every block dev in output of 'info snapshots'

Currently, the output of 'info snapshots' shows fully available snapshots.
It's opaque, hides some snapshot information to users. It's not convenient
if users want to know more about all of snapshot information on every block
device via monitor.

Follow Kevin's and Max's proposals, The patch makes the output more detailed:
(qemu) info snapshots
List of snapshots present on all disks:
 ID        TAG                 VM SIZE                DATE       VM CLOCK
 --        checkpoint-1           165M 2016-05-22 16:58:07   00:02:06.813

List of partial (non-loadable) snapshots on 'drive_image1':
 ID        TAG                 VM SIZE                DATE       VM CLOCK
 1         snap1                     0 2016-05-22 16:57:31   00:01:30.567

Signed-off-by: Lin Ma <lma@suse.com>
Message-id: 1467869164-26688-3-git-send-email-lma@suse.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
