usbredir: free vm_change_state_handler in usbredir destroy dispatch

In usbredir destroy dispatch function, it doesn't free the vm change
state handler once registered in usbredir_realize function. This will
lead a memory leak issue. This patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 58216976.d0236b0a.77b99.bcd6@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
