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
a4afa548.

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>