block: print the server key type and fingerprint on failure

block: print the server key type and fingerprint on failure

When validating the server key fingerprint fails, it is difficult for
the user to know what they got wrong. The fingerprint accepted by QEMU
is received in a different format than OpenSSH displays. There can also
be keys for multiple different ciphers in known_hosts. It may not be
obvious which cipher QEMU will use and whether it will be the same
as OpenSSH. Address this by printing the server key type and its
corresponding fingerprint in the format QEMU accepts.

Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>