utils_test: Add two functions for libvirt tests

Function to check virsh commands exit status.
Function to check interface for virsh iface related tests.

Signed-off-by: Yanbing Du <ydu@redhat.com>

Conflicts:
	virttest/utils_test/libvirt.py
virttest.virsh: Add wrappers for iface related commands

Signed-off-by: Yanbing Du <ydu@redhat.com>
Typo fix

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1587 from will-Do/iface

Add iface related commands wrappers and libvirt common test functions