exec: Pass RAMBlock pointer to qemu_ram_free

The only caller now knows exactly which RAMBlock to free, so it's not
necessary to do the lookup.

Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1456813104-25902-6-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>