guest: arm kvm should use host-passthrough, not model=host

Seems to work with libvirt, not sure if it was ever broken as the comment
suggested.

tests/cli-test-xml/compare/virt-install-aarch64-kvm-import.xml

@@ -13,9 +13,7 @@
     <acpi/>
     <apic/>
   </features>
-  <cpu mode="custom" match="exact">
-    <model>host</model>
-  </cpu>
+  <cpu mode="host-passthrough"/>
   <clock offset="utc"/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>

tests/cli-test-xml/compare/virt-install-arm-kvm-import.xml

@@ -12,9 +12,7 @@
     <acpi/>
     <apic/>
   </features>
-  <cpu mode="custom" match="exact">
-    <model>host</model>
-  </cpu>
+  <cpu mode="host-passthrough"/>
   <clock offset="utc"/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>

virtinst/guest.py

@@ -795,9 +795,7 @@ class Guest(XMLBuilder):
             return
 
         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"
+            self.cpu.mode = self.cpu.SPECIAL_MODE_HOST_PASSTHROUGH
 
         elif self.os.is_arm64() and self.os.is_arm_machvirt():
             # -M virt defaults to a 32bit CPU, even if using aarch64