Monitor: Return before exiting with 'quit'

Monitor: Return before exiting with 'quit'

This is a new version of the (now reverted) following commit:

0e8d2b5575938b8876a3c4bb66ee13c5d306fb6d

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 making do_quit() use
qemu_system_shutdown_request(), so that we exit gracefully.

Thanks to Paolo Bonzini <pbonzini@redhat.com> for suggesting
this solution.

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