Monitor: Return before exiting with 'quit'

The 'quit' Monitor command (implemented by do_quit()) calls
exit() directly, this is problematic under QMP because QEMU
exits before having a chance to send the ok response.

Clients don't know if QEMU exited because of a problem or
because the 'quit' command has been executed.

This commit fixes that by moving the exit() call to the main
loop, so that do_quit() requests the system to quit, instead
of calling exit() directly.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
