char: fold qemu_chr_set_handlers in qemu_chr_fe_set_handlers

qemu_chr_add_handlers*() have been removed in previous change, so the
common qemu_chr_set_handlers() is no longer needed.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-17-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>