ixv(4): Add missing IFLIB_IS_VF flag in iflib shared ctx

From Krzysztof:
The driver built as KLD cannot be unloaded, if this flag is not set.

Submitted by: Krzysztof Galazka <krzysztof.galazka@intel.com>
Reviewed by: shurd@, erj@
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D19402