libvirt.tests: Default to use unsafe for migration testcases.

If there is not 'cache=none' for vm disks, migration will be failed:
error: Unsafe migration: Migration may lead to data corruption if disks use cache != none
So for those cases that do not care about safe, we can pass --unsafe by default.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
migration.tests: Limit some tests to Linux only.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
test.migrate_multi_vms: Check connectivity during migration.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #386 from yumingfei/migration_unsafe_and_windows

tests.migration: Several enhancements.