char: fix ctrl-a b not working

char: fix ctrl-a b not working

CharDriverState.be should be updated to point to the current
associated backend.

Fix the regression introduced in the "mux" chardev from commit
a4afa548fc6dd9842ed86639b4d37d4d1c4ad480.

https://bugs.launchpad.net/bugs/1654137

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170110110621.15287-1-marcandre.lureau@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit fb5e19d2e1472e96d72d5e4d89c20033f8ab345c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>