ui: use "vc" chardev for dbus, gtk & spice-app

Those display have their own implementation of "vc" chardev, which
doesn't use pixman. They also don't implement the width/height/cols/rows
options, so qemu_display_get_vc() should return a compatible argument.

This patch was meant to be with the pixman series, when the "vc" field
was introduced. It fixes a regression where VC are created on the
tty (or null) instead of the display own "vc" implementation.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>