libvirt-tests: Add test case for migrate-compcache

Test command: migrate-compcache <domain> [--size <number>]

* Run migrate-compcache command and check return code

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #45 from Hao-Liu/migrate_compcache

libvirt-tests: Add test case for migrate-compcache