bhyve: verify OpRegion size

If the OpRegion size doesn't match the size of our igd_opregion struct, it's
using a different layout than we're expecting. To avoid strange issues, we
should exit hard. If we see any devices in the field with a different OpRegion
size, we can analyse it and fix it accordingly.

Reviewed by:		markj
MFC after:		1 week
Sponsored by:		Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D45335

usr.sbin/bhyve/amd64/pci_gvt-d.c

@@ -222,6 +222,13 @@ gvt_d_setup_opregion(struct pci_devinst *const pi)
 	opregion->len = header->size * KB;
 	munmap(header, sizeof(*header));
 
+	if (opregion->len != sizeof(struct igd_opregion)) {
+		warnx("%s: Invalid OpRegion size of 0x%lx", __func__,
+		    opregion->len);
+		close(memfd);
+		return (-1);
+	}
+
 	opregion->hva = mmap(NULL, opregion->len, PROT_READ, MAP_SHARED, memfd,
 	    opregion->hpa);
 	if (opregion->hva == MAP_FAILED) {