librbd/migration: make SourceSpecBuilder::parse_source_spec() static

librbd/migration: make SourceSpecBuilder::parse_source_spec() static

In preparation for divorcing NativeFormat from FormatInterface and
changing when/how src_image_ctx is created, make parse_source_spec()
independent of src_image_ctx.  The "invalid source-spec JSON" error is
duplicated by the "failed to parse migration source-spec" error, so
just get rid of the former to spare having to pass CephContext to
parse_source_spec().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>