docs: Document limitation of maximum vcpu count used with <topology>

qemu requires that the topology equals to the maximum vcpu count.
Document this along with the API to set maximum vcpu count and the XML
element.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1426220

docs/formatdomain.html.in

@@ -1401,7 +1401,10 @@
         virtual CPU provided to the guest. Three non-zero values have to be
         given for <code>sockets</code>, <code>cores</code>, and
         <code>threads</code>: total number of CPU sockets, number of cores per
-        socket, and number of threads per core, respectively.</dd>
+        socket, and number of threads per core, respectively. Hypervisors may
+        require that the maximum number of vCPUs specified by the
+        <code>cpus</code> element equals to the number of vcpus resulting
+        from the topology.</dd>
 
       <dt><code>feature</code></dt>
       <dd>The <code>cpu</code> element can contain zero or more

src/libvirt-domain.c

@@ -7010,7 +7010,8 @@ virDomainSetVcpus(virDomainPtr domain, unsigned int nvcpus)
  * CPU limit is altered; generally, this value must be less than or
  * equal to virConnectGetMaxVcpus().  Otherwise, this call affects the
  * current virtual CPU limit, which must be less than or equal to the
- * maximum limit.
+ * maximum limit. Note that hypervisors may not allow changing the maximum
+ * vcpu count if processor topology is specified.
  *
  * If @flags includes VIR_DOMAIN_VCPU_GUEST, then the state of processors is
  * modified inside the guest instead of the hypervisor. This flag can only

tools/virsh.pod

@@ -2477,7 +2477,8 @@ marked as hotpluggable.
 The I<--maximum> flag controls the maximum number of virtual cpus that can
 be hot-plugged the next time the domain is booted.  As such, it must only be
 used with the I<--config> flag, and not with the I<--live> or the I<--current>
-flag.
+flag. Note that it may not be possible to change the maximum vcpu count if
+the processor topology is specified for the guest.
 
 =item B<setvcpu> I<domain> I<vcpulist> [I<--enable>] | [I<--disable>]
 [[I<--live>] [I<--config>] | [I<--current>]]