qemu: process: SEV: Relabel guest owner's SEV files created before start

Before launching a SEV guest we take the base64-encoded guest owner's
data specified in launchSecurity and create files with the same content
under /var/lib/libvirt/qemu/<domain>. The reason for this is that we
need to pass these files on to QEMU which then uses them to communicate
with the SEV firmware, except when it doesn't have permissions to open
those files since we don't relabel them.

https://bugzilla.redhat.com/show_bug.cgi?id=1658112

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>