Force maxmem=memory for HVM with PCI devices

Xen do not support other cases at all ("PCI device assignment for HVM
guest failed due to PoD enabled", PoD means "populate on demand").

templates/libvirt/xen.xml

@@ -2,7 +2,11 @@
     {% block basic %}
         <name>{{ vm.name }}</name>
         <uuid>{{ vm.uuid }}</uuid>
-        <memory unit="MiB">{{ vm.maxmem }}</memory>
+        {% if vm.hvm and vm.devices['pci'].persistent() | list %}
+            <memory unit="MiB">{{ vm.memory }}</memory>
+        {% else %}
+            <memory unit="MiB">{{ vm.maxmem }}</memory>
+        {% endif %}
         <currentMemory unit="MiB">{{ vm.memory }}</currentMemory>
         <vcpu placement="static">{{ vm.vcpus }}</vcpu>
     {% endblock %}