libvirt.tests: add test cases for VM migration with IPv6 and SSH, TCP, TLS transports

This PR is based on PR1875, PR1882, PR1884 and PR1888.

[root@client virt-test]# ./run -t libvirt --tests type_specific.io-github-autotest-libvirt.virsh.migration_with_ssh.positive_testing.ssh_static_ipv6 --no-downloads --keep-image --keep-image-between-tests
SETUP: PASS (0.30 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /home/ajia/Workspace/virt-test/logs/run-2014-11-14-14.27.22/debug.log
TESTS: 1
(1/1) type_specific.io-github-autotest-libvirt.virsh.migration_with_ssh.positive_testing.ssh_static_ipv6: PASS (31.43 s)
TOTAL TIME: 31.48 s
TESTS PASSED: 1
TESTS FAILED: 0
SUCCESS RATE: 100.00 %
[root@client virt-test]# ./run -t libvirt --tests type_specific.io-github-autotest-libvirt.virsh.migration_with_tcp.positive_testing.tcp_static_ipv6 --no-downloads --keep-image --keep-image-between-tests
SETUP: PASS (0.30 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /home/ajia/Workspace/virt-test/logs/run-2014-11-14-14.28.06/debug.log
TESTS: 1
(1/1) type_specific.io-github-autotest-libvirt.virsh.migration_with_tcp.positive_testing.tcp_static_ipv6: PASS (37.89 s)
TOTAL TIME: 37.93 s
TESTS PASSED: 1
TESTS FAILED: 0
SUCCESS RATE: 100.00 %
[root@client virt-test]# ./run -t libvirt --tests type_specific.io-github-autotest-libvirt.virsh.migration_with_tls.positive_testing.tls_static_ipv6 --no-downloads --keep-image --keep-image-between-tests
SETUP: PASS (0.30 s)
DATA DIR: /var/lib/virt_test
DEBUG LOG: /home/ajia/Workspace/virt-test/logs/run-2014-11-14-14.29.01/debug.log
TESTS: 1
(1/1) type_specific.io-github-autotest-libvirt.virsh.migration_with_tls.positive_testing.tls_static_ipv6: PASS (58.18 s)
TOTAL TIME: 58.23 s
TESTS PASSED: 1
TESTS FAILED: 0
SUCCESS RATE: 100.00 %

Signed-off-by: Alex Jia <ajia@redhat.com>
Why not use virsh.destroy(..., uri=remote_uri) to do this job?

BTW, the following steps may be skipped when session is broken.
How about checking and getting session in 'try' block?

So as vmxml_backup.sync().