BLK: added request flag for special VSL requests

Some VSL requests are not meant to be managed by the core VSL
system - for example, KV requests are configured outside the core
VSL system and require no additional handling prior to processing,
nor the usual post-request VSL-specific processing.
This flag identifies such requests such that VSL can skip them.
NVMe: removed unused function
vsl: read_rq and write_rq respect pass-through requests.

Not all requests require further processing by VSL before being
forwarded to the backing device - now the default implementations
of read and write request for a vsl target type respect this fact.
vsl: vsl_complete_request respects requests marked pass-through

Not all requests require further processing by the VSL core.
Requests which are already configured can be marked as 'pass
through' to indicate this. This patch ensures vsl_complete_request
respects such requests.
vsl: Restructuring header file to accommodate KV-capabilities.