hyperv: propagate detailed error from Hyper-V permission checks

Change isHyperVAdminMember from returning a bool to returning an error so that callers surface the specific failure reason — particularly distinguishing "not in the group" from "in the group but session not updated" (new ErrHypervUserSessionNotUpdated sentinel). This gives users actionable guidance instead of a generic permission error.

Signed-off-by: lstocchi <lstocchi@redhat.com>