trap signals for "-serial mon:stdio"

trap signals for "-serial mon:stdio"

With mon:stdio you can exit the VM by switching to the monitor and
sending the "quit" command.  It is then useful to pass Ctrl-C to the
VM instead of exiting.

This in turn lets us stop tying the default signal handling behavior
to -nographic, removing gratuitous differences between "-display none"
and "-nographic".

This patch changes behavior for "-display none -serial mon:stdio", as
expected, but not for "-display none -serial stdio".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1372868986-25988-1-git-send-email-mjt@msgid.tls.msk.ru
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>