Rewrite the implementation of the ssh block driver to use libssh instead
of libssh2.  The libssh library has various advantages over libssh2:
- easier API for authentication (for example for using ssh-agent)
- easier API for known_hosts handling
- supports newer types of keys in known_hosts

Use APIs/features available in libssh 0.8 conditionally, to support
older versions (which are not recommended though).

Adjust the iotest 207 according to the different error message, and to
find the default key type for localhost (to properly compare the
fingerprint with).
Contributed-by: NMax Reitz <mreitz@redhat.com>

Adjust the various Docker/Travis scripts to use libssh when available
instead of libssh2. The mingw/mxe testing is dropped for now, as there
are no packages for it.
Signed-off-by: NPino Toscano <ptoscano@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: NAlex Bennée <alex.bennee@linaro.org>
Message-id: 20190620200840.17655-1-ptoscano@redhat.com
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 5873173.t2JhDm7DL7@lindworm.usersys.redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>