char: start converting mux driver to use CharBackend

Start using qemu_chr_fe* CharBackend functions:
initialize a CharBackend and use qemu_chr_fe_set_handlers().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-11-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
