ui/gtk: move draw_submitted from QemuDmaBuf to VirtualGfxConsole

The draw_submitted flag on QemuDmaBuf is only used by the GTK display
backend to gate the draw pipeline and prevent gl_block refcount
imbalance when the guest submits frames faster than GTK draws them. It
is not an intrinsic property of the DMA-BUF but transient display
listener state.

Move it to VirtualGfxConsole, replacing the getter/setter calls with
direct struct member access. Reset the flag when the active dmabuf is
released in gd_gl_release_dmabuf().

Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>