spice: add SASL support

Turn on SASL support by appending "sasl" to the spice arguments, which
requires that the client use SASL to authenticate with the spice.  The
exact choice of authentication method used is controlled from the
system / user's SASL configuration file for the 'qemu' service. This
is typically found in /etc/sasl2/qemu.conf. If running QEMU as an
unprivileged user, an environment variable SASL_CONF_PATH can be used
to make it search alternate locations for the service config.  While
some SASL auth methods can also provide data encryption (eg GSSAPI),
it is recommended that SASL always be combined with the 'tls' and
'x509' settings to enable use of SSL and server certificates. This
ensures a data encryption preventing compromise of authentication
credentials.

It requires support from spice 0.8.1.

[ kraxel: moved spell fix to separate commit ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
