The previous commit demonstrated documentation and code disagree on
parsing of ',' in the value of an implied key.  Fix the code to match
the documentation.

This breaks uses of keyval_parse() that pass an implied key and accept
a value containing ','.  None of the existing uses does:

* audiodev: implied key "driver" is enum AudiodevDriver, none of the
  values contains ','

* display: implied key "type" is enum DisplayType, none of the values
  contains ','

* blockdev: implied key "driver is enum BlockdevDriver, none of the
  values contains ','

* export: implied key "type" is enum BlockExportType, none of the
  values contains ','

* monitor: implied key "mode" is enum MonitorMode, none of the values
  contains ','

* nbd-server: no implied key.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201011073505.1185335-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>