When performing an install, it's common for tooling such as virt-install
to remove the install kernel/initrd once they are successfully booted and
the domain has been redefined to boot without them. After the installation
is complete and the domain is rebooted/shutdown, the DAC and selinux
security drivers attempt to restore labels on the now deleted files. It's
harmles wrt functionality, but results in error messages such as

Mar 08 12:40:37 virtqemud[5639]: internal error: child reported (status=125): unable to stat: /var/lib/libvirt/boot/vir>
Mar 08 12:40:37 virtqemud[5639]: unable to stat: /var/lib/libvirt/boot/virtinst-yvp19moo-linux: No such file or directo>
Mar 08 12:40:37 virtqemud[5639]: Unable to run security manager transaction

Add a check for file existence to the virSecurity*RestoreFileLabel functions,
and avoid relabeling if the file is no longer available. Skipping the restore
caused failures in qemusecuritytest, which mocks s...