QMP: Move RESET event into qemu_system_reset()

Nothing will change as that function is currently only called by
the main loop code, but it's the right place for the RESET event,
as it's where the reset is actually performed.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
