Widening API change - accept empty path for virDomainBlockStats

And provide domain summary stat in that case, for lxc backend.
Use case is a container inheriting all devices from the host,
e.g. when doing application containerization.

tools/virsh-domain-monitor.c

@@ -888,7 +888,7 @@ static const vshCmdOptDef opts_domblkstat[] = {
     },
     {.name = "device",
      .type = VSH_OT_DATA,
-     .flags = VSH_OFLAG_REQ,
+     .flags = VSH_OFLAG_EMPTY_OK,
      .help = N_("block device")
     },
     {.name = "human",
@@ -954,9 +954,16 @@ cmdDomblkstat(vshControl *ctl, const vshCmd *cmd)
     if (!(dom = vshCommandOptDomain(ctl, cmd, &name)))
         return false;
 
+    /* device argument is optional now. if it's missing, supply empty
+       string to denote 'all devices'. A NULL device arg would violate
+       API contract.
+     */
     if (vshCommandOptStringReq(ctl, cmd, "device", &device) < 0)
         goto cleanup;
 
+    if (!device)
+        device = "";
+
     rc = virDomainBlockStatsFlags(dom, device, NULL, &nparams, 0);
 
     /* It might fail when virDomainBlockStatsFlags is not