Fix do_commit() behavior

Fix do_commit() behavior

Commit 751c6a17042b5d011013d6963c0505d671cf708e changed the monitor's
'commit' command to this behavior:

1. Any string you type as argument will cause do_commit() to
call bdrv_commit() to all devices

2. If you enter a device name, it will be the only one ignored
by do_commit() :)

The fix is to call bdrv_commit() to the specified device only and
ignore the others (when 'all' is not specified).

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
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