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

@@ -0,0 +1,90 @@
+<domain type='kvm'>
+  <name>Linux5.4</name>
+  <memory unit='KiB'>4194304</memory>
+  <description>QEMU Virtual machine model optimized for Linux guests running at least kernel 5.4. Secure boot is disabled</description>
+  <memory unit="KiB">4194304</memory>
+  <currentMemory unit="KiB">1048576</currentMemory>
+  <memoryBacking>
+    <source type="memfd"/>
+    <access mode="shared"/>
+  </memoryBacking>
+  <vcpu placement='static'>4</vcpu>
+  <os>
+    <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/>
+    <apic/>
+    <vmport state='off'/>
+  </features>
+  <cpu mode="host-passthrough" check="none" migratable="on">
+    <topology sockets="1" dies="1" cores="2" threads="2"/>
+  </cpu>
+  <clock offset='utc'>
+    <timer name='rtc' tickpolicy='catchup'/>
+    <timer name='pit' tickpolicy='delay'/>
+    <timer name='hpet' present='no'/>
+  </clock>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <pm>
+    <suspend-to-mem enabled='no'/>
+    <suspend-to-disk enabled='no'/>
+  </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">
+      <model type="virtio"/>
+      <source network="default"/>
+    </interface>
+    <console type='pty'>
+      <target type='virtio' port='0'/>
+    </console>
+    <channel type="spicevmc">
+      <target type="virtio" name="com.redhat.spice.0"/>
+    <address type="virtio-serial" controller="0" bus="0" port="1"/>
+    </channel>
+    <channel type='unix'>
+      <target type='virtio' name='org.qemu.guest_agent.0'/>
+      <address type='virtio-serial' controller='0' bus='0' port='2'/>
+    </channel>
+    <input type='keyboard' bus='virtio'>
+    </input>
+    <input type='tablet' bus='virtio'>
+    </input>
+    <tpm model="tpm-crb">
+      <backend type="emulator" version="2.0"/>
+    </tpm>
+    <graphics type="spice">
+      <listen type="none"/>
+      <image compression="off"/>
+      <gl enable="yes"/>
+    </graphics>
+    <video>
+      <model type="virtio" heads="1" primary="yes">
+      <acceleration accel3d="yes"/>
+      </model>
+    </video>
+    <memballoon model="virtio"/>
+    <rng model='virtio'>
+      <backend model='random'>/dev/urandom</backend>
+    </rng>
+    <filesystem type="mount" accessmode="passthrough">
+      <driver type="virtiofs"/>
+      <source dir="/mnt/"/>
+      <target dir="share"/>
+    </filesystem>
+  </devices>
+</domain>
+