build: remove MCS SELinux labels from built image
If we don't do this we end up with an image with MCS labels which means
that a subsequent run of a container (i.e. `coreos-assembler run`) will
fail.
With this we go from:
```
system_u:object_r:container_file_t:s0:c255,c505
```
to:
```
system_u:object_r:container_file_t:s0
```
Fixes
@@ -217,6 +217,9 @@ fi
/usr/lib/coreos-assembler/gf-anaconda-cleanup "$(pwd)"/"${img_base}"
/usr/lib/coreos-assembler/gf-oemid "$(pwd)"/"${img_base}" "$(pwd)"/"${img_qemu}" qemu
set +x
+# Clear the MCS SELinux labels
+# See https://github.com/coreos/coreos-assembler/issues/292
+chcon -vl s0 "${img_qemu}"
# make a version-less symlink to have a stable path
ln -s "${img_qemu}" "${name}"-qemu.qcow2
