ice: introduce VF accessor functions

Before we switch the VF data structure storage mechanism to a hash,
introduce new accessor functions to define the new interface.

* ice_get_vf_by_id is a function used to obtain a reference to a VF from
  the table based on its VF ID
* ice_has_vfs is used to quickly check if any VFs are configured
* ice_get_num_vfs is used to get an exact count of how many VFs are
  configured

We can drop the old ice_validate_vf_id function, since every caller was
just going to immediately access the VF table to get a reference
anyways. This way we simply use the single ice_get_vf_by_id to both
validate the VF ID is within range and that there exists a VF with that
ID.

This change enables us to more easily convert the codebase to the hash
table since most callers now properly use the interface.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>