xenapi: refactor - extract get_stream_funct_for

This change extracts the get_stream_funct_for function. As we would like
to support tar.gz compressed raw images, the stream function for them
should be different from the plain ones.

related to blueprint xenapi-supported-image-import-export

Change-Id: I27681da069ba2cd23670ef41f54ed9ac5af31ae6
Merge "xenapi: refactor - extract get_stream_funct_for"