console: make QMP/HMP screendump run in coroutine

console: make QMP/HMP screendump run in coroutine

Thanks to the monitors' coroutine support (merge commit b7092cda1b3),
the screendump handler can trigger a graphic_hw_update(), yield and let
the main loop run until update is done. Then the handler is resumed, and
ppm_save() will write the screen image to disk in the coroutine context.

The IO is still blocking though, as the file is set blocking so far,
this could be addressed by some future change (with other caveats).

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1230527

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20201027133602.3038018-4-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>