New api to read and check specific label in context of the given file

[bssrikanth]

Currently there are only methods to get and set the context of the
given file. There are usecases to check for specific labels in the
context of the file. This commit introduces new method to do the same.

Signed-off-by: Srikanth Aithal sraithal@linux.vnet.ibm.com

[bssrikanth]

test results of case when context of filename is returned empty:

2019-08-19 06:41:23,711 utils_selinux    L0296 WARNI| Context of shared filename '/var/lib/libvirt/images/avocado_1t2CsG/test.iso' is None
2019-08-19 06:41:23,759 libvirt_vm       L2140 DEBUG| Destroying VM
2019-08-19 06:41:24,291 libvirt_vm       L2175 WARNI| Requested MAC address release from persistent vm vm1. Ignoring.
2019-08-19 06:41:24,392 virsh            L1434 DEBUG| Undefine VM vm1
2019-08-19 06:41:24,411 virsh            L1418 DEBUG| Define VM from /var/tmp/xml_utils_temp_JiTBLw.xml
2019-08-19 06:41:24,581 virsh            L1434 DEBUG| Undefine VM vm2
2019-08-19 06:41:24,603 virsh            L1418 DEBUG| Define VM from /var/tmp/xml_utils_temp_bOb5BA.xml
2019-08-19 06:41:24,683 stacktrace       L0041 ERROR|
2019-08-19 06:41:24,683 stacktrace       L0044 ERROR| Reproduced traceback from: /usr/local/lib/python2.7/dist-packages/avocado_plugins_vt-69.0-py2.7.egg/avocado_vt/test.py:449
2019-08-19 06:41:24,683 stacktrace       L0047 ERROR| Traceback (most recent call last):
2019-08-19 06:41:24,683 stacktrace       L0047 ERROR|   File "/home/srikanth/avocado-fvt-wrapper/data/avocado-vt/test-providers.d/downloads/powerkvm-libvirt/libvirt/tests/src/virtual_disks/virtual_dd
isks_multivms.py", line 323, in run
2019-08-19 06:41:24,683 stacktrace       L0047 ERROR|     test.fail("Context of shared iso is not expected.")
2019-08-19 06:41:24,683 stacktrace       L0047 ERROR|   File "/usr/local/lib/python2.7/dist-packages/avocado_framework-68.0-py2.7.egg/avocado/core/test.py", line 1025, in fail
2019-08-19 06:41:24,684 stacktrace       L0047 ERROR|     raise exceptions.TestFail(message)
2019-08-19 06:41:24,684 stacktrace       L0047 ERROR| TestFail: Context of shared iso is not expected.
2019-08-19 06:41:24,684 stacktrace       L0048 ERROR|

[bssrikanth]

@xutian thanks for reviewing. I have updated right version here. Results are as well posted @ autotest/tp-libvirt#2279

[balamuruhans]

Looks good to me,

[balamuruhans]

return label not in se_label

[balamuruhans]

return bool(se_label)

[sathnaga]

Let's merge this, we can take minor changes as separate PR.