qemu-ga: Optimize freeze-hook script logic of logging error

Make sure the error log of fsfreeze hooks
when freeze/thaw/snapshot could be logged
to system logs if the default logfile of
qga can't be written or other situations

Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Yan Vugenfirer <yvugenfi@redhat.com>
Signed-off-by: Dehan Meng <demeng@redhat.com>

Jira: https://issues.redhat.com/browse/RHEL-52278

Signed-off-by: Dehan Meng demeng@redhat.com