In xen_hvm_change_state_handler(), we can pass 'opaque' with type cast
to xen_main_loop_prepare() directly, there's no need to use additional
variable for it.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>