qemu: minor refactor of video device string handling

qemu: minor refactor of video device string handling

In preparation for adding the bochs display device, refactor the logic
so that each branch handles a single device type and checks its
parameters within that branch. In this case VGA and VMVGA are still
grouped into the same branch since they share device-specific parameter
names.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>