char: move front end handlers in CharBackend

Since the hanlders are associated with a CharBackend, rather than the
CharDriverState, it is more appropriate to store in CharBackend. This
avoids the handler copy dance in qemu_chr_fe_set_handlers() then
mux_chr_update_read_handler(), by storing the CharBackend pointer
directly.

Also a mux CharDriver should go through mux->backends[focused], since
chr->be will stay NULL. Before that, it was possible to call
chr->handler by mistake with surprising results, for ex through
qemu_chr_be_can_write(), which would result in calling the last set
handler front end, not the one with focus.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-22-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>