rdma: Fix qemu crash when IPv6 address is used for migration

Qemu crashes when IPv6 address is specified for migration and access
to any RDMA uverbs device available on the system is blocked using cgroups.
Fix the crash by checking the return value of ibv_open_device routine.
Signed-off-by: NMeghana Cheripady <meghana.cheripady@avagotech.com>
Signed-off-by: NPadmanabh Ratnakar <padmanabh.ratnakar@avagotech.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration/rdma.c

@@ -790,6 +790,13 @@ static int qemu_rdma_broken_ipv6_kernel(Error **errp, struct ibv_context *verbs)
 
         for (x = 0; x < num_devices; x++) {
             verbs = ibv_open_device(dev_list[x]);
+            if (!verbs) {
+                if (errno == EPERM) {
+                    continue;
+                } else {
+                    return -EINVAL;
+                }
+            }
 
             if (ibv_query_port(verbs, 1, &port_attr)) {
                 ibv_close_device(verbs);