libifconfig: Add a native SR-IOV VF status query

[kbowling]

Summary

Request RTEXT_FILTER_VF through route Netlink and parse the common VF
status schema into typed public structures.  Preserve per-field
presence using IFLAF_VF_* attribute numbers as mask bit indices so
callers can distinguish omitted values from false or zero.

Validate required VF indices, repeated driver namespaces, and their
versioned typed fields while allowing unknown optional attributes.

[melifaro]

lib/libifconfig/libifconfig.h

217

Those should come from the kernel headers

249

if we really want to have this snl passthru we should use definitions from the snl header

276

Why can't if_vf_info from snl be used here?

[kbowling]

Maps every IFLAF_VF_* presence bit to the stable public IFCONFIG_VF_F_* flags

[kbowling]

Use netlink/route/interface.h