qemu: Fix CPU=power8 to work properly with KVM

You can't use -cpu power8 with KVM on a Power9, it errors with:

  If you're using kvm-hv.ko, only "-cpu host" is possible

Instead we need to specify the "max-cpu-compat" property as a machine
capability.

Author: mpe

lib/qemu.py

@@ -65,8 +65,14 @@ def configure_from_args(self, args):
             self.interactive = True
 
     def apply_defaults(self):
-        if self.machine_is('pseries') and self.accel == 'tcg':
-            self.machine_caps += ['cap-htm=off']
+        if self.machine_is('pseries'):
+            if self.accel == 'tcg':
+                self.machine_caps += ['cap-htm=off']
+
+            if self.cpu and self.accel == 'kvm':
+                if self.cpu != 'host':
+                    self.machine_caps += ['max-cpu-compat=%s' % self.cpu.lower()]
+                self.cpu = None
 
         if self.cpuinfo is None:
             if self.machine_is('pseries'):