qapi: Add device ID and head parameters to screendump

qapi: Add device ID and head parameters to screendump

QEMU's screendump command can only take dumps from the primary display.
When using multiple VGA cards, there is no way to get a dump from a
secondary card or other display heads yet. So let's add a 'device' and
a 'head' parameter to the HMP and QMP commands to be able to specify
alternative devices and heads with the screendump command, too.

Reviewed-by: Daniel P. Berrangé <[email protected]>
Acked-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Gerd Hoffmann <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 1520267868[email protected]
Signed-off-by: Gerd Hoffmann <[email protected]>