spice: abort on invalid streaming cmdline params

When parsing its command line parameters, spice aborts when it
finds unexpected values, except for the 'streaming-video' option.
This happens because the parsing of the parameters for this option
is done using the 'name2enum' helper, which does not error out
on unknown values. Using the 'parse_name' helper makes sure we
error out in this case. Looking at git history, the use of
'name2enum' instead of 'parse_name' seems to have been an oversight,
so let's change to that now.

Fixes rhbz#831708

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