mux: split mux_chr_update_read_handler()

mux: split mux_chr_update_read_handler()

Make qemu_chr_add_handlers_full() aware of mux handling. This allows
introduction of a tag associated with the fe handlers and a
qemu_chr_set_handlers() function to set the handler for a particular
tag. That will allow to get rid of qemu_chr_add_handlers*() in later
changes, in favor of qemu_chr_fe_set_handler().

To this end, chr_update_read_handler callback is enhanced with a tag
argument, and mux_chr_update_read_handler() is splitted in new
functions: mux_chr_new_handler_tag(), mux_chr_set_handlers(),
mux_set_focus().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-9-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>