qemu: migration: Improve p2p error if we can't open conn

By actually showing the Open() error to the user
(cherry picked from commit 5751fc4f)

src/qemu/qemu_migration.c

@@ -3512,7 +3512,8 @@ static int doPeer2PeerMigrate(virQEMUDriverPtr driver,
     qemuDomainObjExitRemote(vm);
     if (dconn == NULL) {
         virReportError(VIR_ERR_OPERATION_FAILED,
-                       _("Failed to connect to remote libvirt URI %s"), dconnuri);
+                       _("Failed to connect to remote libvirt URI %s: %s"),
+                       dconnuri, virGetLastErrorMessage());
         virObjectUnref(cfg);
         return -1;
     }