gdbstub: Allow re-instantiation (Jan Kiszka)

[ Note: depends on char closing fixes ]

Properly clean up the gdbstub when the user tries to re-open it
(possibly under a different address). Moreover, allow to shut it down
from the monitor via 'gdbserver none'.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6913 c046a42c-6fe2-441c-8c8c-71466251a162