iotests: Add 'check -ssh' option to test Secure Shell block device.

Note in order to run these tests on ssh, you must be running a local
ssh daemon, and that daemon must accept loopback connections, and
ssh-agent has to be set up to allow logins on the local daemon.  In
other words, the following command should just work without demanding
any passphrase:

 ssh localhost

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
