qemu: Prefer PNG for domain screenshots

qemu: Prefer PNG for domain screenshots

Historically, QEMU took screenshots in PPM. While this might use
to be popular format, as of v7.1.0-rc0~125^2~6 it is possible to
take screenshots in PNG. This is more popular and renders almost
everywhere, which is not the case for PPM (for instance, modern
browsers do not render it).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>