libvirt.tests: Support default value for --timeout in .cfg files for virsh migrate.

Migrate with stress may cost more than the old default value for
"virsh migrate --timeout", which was 60 and was assigned in the .py files.
This patch provides "virsh_migrate_timeout" param in .cfg files.
Merge pull request #315 from zengdh/migrate-timeout

libvirt.tests: Support value for --timeout in .cfg files for virsh migrate.