spice-display: replace private lock with qemu mutex.

qemu_spice_create_update() must aquire the global qemu mutex to
make sure DisplayState doesn't change while we are accessing it.

Once this is in place the private lock is pretty pointless as
everything it protects is covered by the global qemu mutex now.
Drop it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
