tests/vm: Take the J=x setting into account for the vm-boot-ssh targets, too

For testing whether the VMs can deal with multiple CPUs correctly,
it is useful to be able to use the "J=<cpus>" setting for the
vm-boot-ssh targets, too.

Message-Id: <20190726100207.19112-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>