qemu: hotplug: print correct vcpu when validating hot(un)plug config

The error message would contain first vcpu id after the list of vcpus
selected for modification. To print the proper vcpu id remember the
first vcpu selected to be modified.

src/qemu/qemu_hotplug.c

@@ -5866,6 +5866,7 @@ qemuDomainVcpuValidateConfig(virDomainDefPtr def,
     virDomainVcpuDefPtr vcpu;
     size_t maxvcpus = virDomainDefGetVcpusMax(def);
     ssize_t next;
+    ssize_t firstvcpu = -1;
 
     /* vcpu 0 can't be disabled */
     if (!state && virBitmapIsBitSet(map, 0)) {
@@ -5880,13 +5881,17 @@ qemuDomainVcpuValidateConfig(virDomainDefPtr def,
             continue;
 
         /* skip vcpus being modified */
-        if (virBitmapIsBitSet(map, next))
+        if (virBitmapIsBitSet(map, next)) {
+            if (firstvcpu < 0)
+                firstvcpu = next;
+
             continue;
+        }
 
         if (vcpu->online && vcpu->hotpluggable == VIR_TRISTATE_BOOL_NO) {
             virReportError(VIR_ERR_INVALID_ARG,
                            _("vcpu '%zd' can't be modified as it is followed "
-                             "by non-hotpluggable online vcpus"), next);
+                             "by non-hotpluggable online vcpus"), firstvcpu);
             return -1;
         }
     }