idpf: add idpf_virtchnl.h

idpf.h is quite heavy. We can reduce the burden a fair bit by
introducing an idpf_virtchnl.h file. This mostly just moves function
declarations but there are many of them. This also makes an attempt to
group those declarations in a way that makes some sense instead of
mishmashed.

Suggested-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>