selinux_seclabel_per_device:Fix UnboundLocalError

[chloerh]

Test result:

 (1/6) type_specific.io-github-autotest-libvirt.svirt.selinux.seclabel.per_device.relabel_no.serial.cold_plug: PASS (7.05 s)
 (2/6) type_specific.io-github-autotest-libvirt.svirt.selinux.seclabel.per_device.relabel_yes.without_label.serial.cold_plug: FAIL: Run '/bin/virsh start avocado-vt-vm1 ' expect fail, but run successfully. (7.71 s)
 (3/6) type_specific.io-github-autotest-libvirt.svirt.selinux.seclabel.per_device.relabel_yes.label_legit.serial.cold_plug:
PASS (7.39 s)
 (4/6) type_specific.io-github-autotest-libvirt.svirt.selinux.seclabel.per_device.relabel_yes.label_MCS.serial.cold_plug:PAASS (7.25 s)
 (5/6) type_specific.io-github-autotest-libvirt.svirt.selinux.seclabel.per_device.relabel_yes.label_default.serial.cold_plug: PASS (7.21 s)
 (6/6) type_specific.io-github-autotest-libvirt.svirt.selinux.seclabel.per_device.relabel_yes.label_invalid_fmt.serial.cold_plug: PASS (7.15 s)
RESULTS    : PASS 5 | ERROR 0 | FAIL 1 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0