Qemu/Xen: Fix early freeing MSIX MMIO memory region

Qemu/Xen: Fix early freeing MSIX MMIO memory region

msix->mmio is added to XenPCIPassthroughState's object as property.
object_finalize_child_property is called for XenPCIPassthroughState's
object, which calls object_property_del_all, which is going to try to
delete msix->mmio. object_finalize_child_property() will access
msix->mmio's obj. But the whole msix struct has already been freed
by xen_pt_msix_delete. This will cause segment fault when msix->mmio
has been overwritten.

This patch is to fix the issue.

Signed-off-by: Lan Tianyu <[email protected]>
Reviewed-by: Stefano Stabellini <[email protected]>
Signed-off-by: Stefano Stabellini <[email protected]>