ice: Calculate guaranteed VSIs per function and use it

Currently we are setting the guar_num_vsi to equal to ICE_MAX_VSI
which is the device limit of 768. This is incorrect and could have
unintended consequences. To fix this use the valid_function's 8-bit
bitmap returned from discovering device capabilities to determine the
guar_num_vsi per function. guar_num_vsi value is then passed on to
pf->num_alloc_vsi.

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>