tests.migration: Several enhancements.

1.
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.
 
2.
migration.tests: Limit some tests to Linux only.

3.
test.migrate_multi_vms: Check connectivity during migration.