qemu: agent: Don't automatically disable CPU0 via guest agent

While CPU0 was made unpluggable in Linux a while ago it's not desirable
to unplug it since some parts of the kernel (suspend-to-ram) still
depend on it.

This patch fixes the vCPU selection code in libvirt so that it will not
be disabled.

src/qemu/qemu_agent.c

@@ -1601,9 +1601,13 @@ qemuAgentUpdateCPUInfo(unsigned int nvcpus,
     size_t i;
     int nonline = 0;
     int nofflinable = 0;
+    ssize_t cpu0 = -1;
 
     /* count the active and offlinable cpus */
     for (i = 0; i < ncpuinfo; i++) {
+        if (cpuinfo[i].id == 0)
+            cpu0 = i;
+
         if (cpuinfo[i].online)
             nonline++;
 
@@ -1618,6 +1622,15 @@ qemuAgentUpdateCPUInfo(unsigned int nvcpus,
         }
     }
 
+    /* CPU0 was made offlinable in linux a while ago, but certain parts (suspend
+     * to ram) of the kernel still don't cope well with that. Make sure that if
+     * all remaining vCPUs are offlinable, vCPU0 will not be selected to be
+     * offlined automatically */
+    if (nofflinable == nonline && cpu0 >= 0 && cpuinfo[cpu0].online) {
+        cpuinfo[cpu0].offlinable = false;
+        nofflinable--;
+    }
+
     /* the guest agent reported less cpus than requested */
     if (nvcpus > ncpuinfo) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",