Added parameter to take screenshot with screendump as PNG

Currently screendump only supports PPM format, which is un-compressed. Added
a "format" parameter to QMP and HMP screendump command to support PNG image
capture using libpng.

QMP example usage:
{ "execute": "screendump", "arguments": { "filename": "/tmp/image",
"format":"png" } }

HMP example usage:
screendump /tmp/image -f png

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/718



Signed-off-by: Kshitij Suri <kshitij.suri@nutanix.com>

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20220408071336.99839-3-kshitij.suri@nutanix.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>