monitor: Rework modal password input (Jan Kiszka)

monitor: Rework modal password input (Jan Kiszka)

Currently, waiting for the user to type in some password blocks the
whole VM because monitor_readline starts its own I/O loop. And this loop
also screws up reading passwords from virtual console.

Patch below fixes the shortcomings by using normal I/O processing also
for waiting on a password. To keep to modal property for the monitor
terminal, the command handler is temporarily replaced by a password
handler and a callback infrastructure is established to process the
result before switching back to command mode.

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@6710 c046a42c-6fe2-441c-8c8c-71466251a162