virttest.env_process: Record qemu pid in screendump folder name.

Sometimes, Two qemu may share same vm.name. So add qemu pid to
screendump folder name.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.qemu_vm: Update clone VM to env in vm.migrate().

Now we do not record cloned VM to env. So we only can catch screendump
for source VM.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.env_process: Encode screendump image to video in postprocess()

Now we Encode screendump image to video in postprocess_vm(), but it could only
encode source VM's image to video in migration.
So move the function to postprocess()

Signed-off-by: Feng Yang <fyang@redhat.com>