block/ssh: Require libssh2 >= 1.2.8.

libssh2 >= 1.2.8 is required to enable this block device (because
that version introduced the libssh2_session_handshake call).

Change the test to use pkg-config exclusively.  If the user requests
--enable-libssh2 and the minimum version is not available, then the
following error is displayed:

  $ ./configure --enable-libssh2

  ERROR: libssh2 >= 1.2.8 required for --enable-libssh2

If --enable-libssh2 is not specified, then the feature is silently
disabled if sufficiently new libssh2 is not available.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>