secdrivers: Relabel firmware config files

secdrivers: Relabel firmware config files

For the case where -fw_cfg uses a file, we need to set the
seclabels on it to allow QEMU the access. While QEMU allows
writing into the file (if specified on the command line), so far
we are enabling reading only and thus we can use read only label
(in case of SELinux).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>