s390x/tcg: initialize machine check queue

Just as for external interrupts and I/O interrupts, we need to
initialize mchk_index during cpu reset.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
