virt: Adds migration tests for dst problem.

1) Migration to read-only space.
2) Migration to low space destination.

Both migration uses exec protocol for migration to file.

qemu-kvm is started like Expect class not only like Tail.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Allows aexcept choose if process should be killed on end of autotest

Signed-off-by: Jiří Župka <jzupka@redhat.com>