mux: fix ctrl-a b again

mux: fix ctrl-a b again

Commit fb5e19d2e1472e96d72d5e4d89c20033f8ab345c originally fixed the
regression, but was inadvertently broken again in merge commit
2d6752d38d8acda.

Fixes:
https://bugs.launchpad.net/qemu/+bug/1654137

Cc: qemu-stable@nongnu.org
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180515152500.19460-3-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit eeaa6715050ed3f9cbedd322220f31570a503217)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>