guest: Set optimal arm CPU defaults

If doing arm kvm, use cpu=host. Otherwise, specify cortex-a57 for aarch64
so we actually get a 64bit cpu

tests/cli-test-xml/compare/virt-install-aarch64-machdefault.xml

@@ -15,6 +15,9 @@
     <apic/>
     <pae/>
   </features>
+  <cpu mode="custom" match="exact">
+    <model>cortex-a57</model>
+  </cpu>
   <clock offset="utc"/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>

tests/cli-test-xml/compare/virt-install-aarch64-machvirt.xml

@@ -15,6 +15,9 @@
     <apic/>
     <pae/>
   </features>
+  <cpu mode="custom" match="exact">
+    <model>cortex-a57</model>
+  </cpu>
   <clock offset="utc"/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>

virtinst/guest.py

@@ -748,19 +748,27 @@ class Guest(XMLBuilder):
     def _set_cpu_defaults(self):
         self.cpu.set_topology_defaults(self.vcpus)
 
-        if (not self.conn.is_test() and not
-            (self.conn.is_qemu() and self.type == "kvm")):
-            return
-        if not self.os.is_x86():
-            return
-        if self.os.arch != self.conn.caps.host.cpu.arch:
-            return
-        if self.cpu.special_mode_was_set:
+        if not self.conn.is_test() and not self.conn.is_qemu():
             return
         if self.cpu.get_xml_config().strip():
+            # User already configured CPU
             return
 
-        self.cpu.set_special_mode(self.x86_cpu_default)
+        if self.os.is_arm_machvirt() and self.type == "kvm":
+            # Should be host-passthrough, but the libvirt support is
+            # incomplete for arm cpu
+            self.cpu.model = "host"
+
+        elif self.os.is_arm64() and self.os.is_arm_machvirt():
+            # -M virt defaults to a 32bit CPU, even if using aarch64
+            self.cpu.model = "cortex-a57"
+
+        elif self.os.is_x86() and self.type == "kvm":
+            if self.os.arch != self.conn.caps.host.cpu.arch:
+                return
+            if self.cpu.special_mode_was_set:
+                return
+            self.cpu.set_special_mode(self.x86_cpu_default)
 
     def _set_feature_defaults(self):
         if self.os.is_container():