tests.migrate: Create a vm automatically for --dname.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
tests.migrate: Fix problems for compcache.

 * Parameter remote uri should be compcache_remote_uri.
 * The showup time of domjob for compcache may be narrow.
   So get job_type and compression cache at same time to avoid miss.
 * Fix a typo.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Pass load_vms instances for stress.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Scsi device supports only qcow2 format.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>