qemuDomainVcpuValidateConfig: Properly initialize 'firstcpu' variable

The loop which checks whether the vcpus are in proper configuration for
the requested hot(un)plug skips the first modified vcpu. This means
that 'firstvcpu' which is used to print the error message in case the
configuration is not suitable would never point to the first modified
vcpu.

In cases such as:

  <vcpu placement='auto' current='5'>8</vcpu>
  <vcpus>
    <vcpu id='0' enabled='yes' hotpluggable='no'/>
    <vcpu id='1' enabled='yes' hotpluggable='no'/>
    <vcpu id='2' enabled='yes' hotpluggable='no'/>
    <vcpu id='3' enabled='yes' hotpluggable='no'/>
    <vcpu id='4' enabled='yes' hotpluggable='no'/>
    <vcpu id='5' enabled='no' hotpluggable='yes'/>
    <vcpu id='6' enabled='no' hotpluggable='yes'/>
    <vcpu id='7' enabled='no' hotpluggable='yes'/>
  </vcpus>

 # virsh setvcpu --config --disable  upstream 1
 error: invalid argument: vcpu '-1' can't be modified as it is followed by non-hotpluggable online vcpus

After this fix the proper vcpu is reported in the error message:

 # virsh setvcpu --config --disable  upstream 1
 error: invalid argument: vcpu '1' can't be modified as it is followed by non-hotpluggable online vcpu

https://bugzilla.redhat.com/show_bug.cgi?id=1611061Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_hotplug.c

@@ -6478,18 +6478,16 @@ qemuDomainVcpuValidateConfig(virDomainDefPtr def,
         return -1;
     }
 
+    firstvcpu = virBitmapNextSetBit(map, -1);
+
     /* non-hotpluggable vcpus need to stay clustered starting from vcpu 0 */
-    for (next = virBitmapNextSetBit(map, -1) + 1; next < maxvcpus; next++) {
+    for (next = firstvcpu + 1; next < maxvcpus; next++) {
         if (!(vcpu = virDomainDefGetVcpu(def, next)))
             continue;
 
         /* skip vcpus being modified */
-        if (virBitmapIsBitSet(map, next)) {
-            if (firstvcpu < 0)
-                firstvcpu = next;
-
+        if (virBitmapIsBitSet(map, next))
             continue;
-        }
 
         if (vcpu->online && vcpu->hotpluggable == VIR_TRISTATE_BOOL_NO) {
             virReportError(VIR_ERR_INVALID_ARG,