block: Split out parse_json_protocol()

The next patch distinguishes options that were explicitly set and
options that were derived. bdrv_fill_option() added options of both
types: Options given by json: syntax should be counted as explicit, but
the rest is derived.

In preparation for the distinction, move json: parse to a separate
function.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>