devices: panic: let libvirt fill in model default

Currently `--panic default` for aarch64 doesn't even request
a `<panic/>` device due to quirky xmlbuilder behavior. Fix that,
but more generally just leave model empty and let libvirt fill it
in for us.

Signed-off-by: Cole Robinson <crobinso@redhat.com>

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

@@ -64,5 +64,6 @@
     <rng model="virtio">
       <backend model="random">/dev/urandom</backend>
     </rng>
+    <panic model="pvpanic"/>
   </devices>
 </domain>

tests/data/cli/compare/virt-install-many-devices.xml

@@ -941,7 +941,7 @@
       </backend>
       <rate bytes="1234" period="1000"/>
     </rng>
-    <panic model="isa">
+    <panic>
       <address type="isa" iobase="0x500" irq="5"/>
     </panic>
     <shmem name="shmem0" role="master">

tests/data/cli/compare/virt-install-singleton-config-2.xml

@@ -91,7 +91,7 @@
     <rng model="virtio">
       <backend model="random">/dev/urandom</backend>
     </rng>
-    <panic model="isa"/>
+    <panic/>
     <vsock model="virtio">
       <cid auto="yes"/>
     </vsock>
@@ -191,7 +191,7 @@
     <rng model="virtio">
       <backend model="random">/dev/urandom</backend>
     </rng>
-    <panic model="isa"/>
+    <panic/>
     <vsock model="virtio">
       <cid auto="yes"/>
     </vsock>