Modify predefined devices

Add virtio memory balloon device, reduce default memory
allocation to 1024MB, disable secureboot,
bump QEMU machine to 7.2, add netboot.iso
and CDROM device

xml/system/linux54.xml

@@ -1,15 +1,18 @@
 <domain type='kvm'>
-  <name>Linux</name>
+  <name>Linux5.4</name>
   <memory unit='KiB'>4194304</memory>
-  <description>System-based Q35 UEFI-based virtual machine optimized for Linux guests using a modern distribution</description>
+  <description>QEMU Virtual machine model optimized for Linux guests running at least kernel 5.4. Secure boot is disabled</description>
-  <currentMemory unit='KiB'>4194304</currentMemory>
+  <memory unit="KiB">4194304</memory>
+  <currentMemory unit="KiB">1048576</currentMemory>
   <memoryBacking>
     <source type="memfd"/>
     <access mode="shared"/>
   </memoryBacking>
   <vcpu placement='static'>4</vcpu>
-  <os firmware="efi">
+  <os>
-    <type arch='x86_64' machine='pc-q35-7.0'>hvm</type>
+    <type arch='x86_64' machine='pc-q35-7.2'>hvm</type>
+    <loader readonly='yes' secure='no' type='pflash'>/usr/share/OVMF/OVMF_CODE.fd</loader>    
+    <boot dev='cdrom'/>
   </os>
   <features>
     <acpi/>
@@ -33,6 +36,12 @@
   </pm>
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <disk type="file" device="cdrom">
+      <driver name="qemu" type="raw"/>
+      <source file="/var/lib/libvirt/isos/netboot.xyz.iso"/>
+      <target dev="sda" bus="sata"/>
+      <readonly/>
+    </disk>
     <controller type='virtio-serial' index='0'>
     </controller>
     <interface type="network">
@@ -67,7 +76,7 @@
       <acceleration accel3d="yes"/>
       </model>
     </video>
-    <memballoon model='none'/>
+    <memballoon model="virtio"/>
     <rng model='virtio'>
       <backend model='random'>/dev/urandom</backend>
     </rng>