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>
utils_test.libvirt: Enhancements to MigrationTest class.

 * Record spent time of migration.
 * Allow to pass options for migration command.
 * Cleanup remote vm by gracefully=False to avoid
   unaccessible vm session.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.load_stress: Fix typo.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.libvirt: Fix bugs of cross migration.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1585 from yumingfei/enhance_migration

utils_test.libvirt: Enhancements to MigrationTest class.
Merge pull request #1587 from will-Do/iface

Add iface related commands wrappers and libvirt common test functions
documentation: .rst fixes

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>