serial: fix memory leak in serial exit

The serial_exit_core function doesn't free some resources.
This can lead memory leak when hotplug and unplug. This
patch avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-Id: <586cb5ab.f31d9d0a.38ac3.acf2@mx.google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
