ixv: Recover E610 Hyper-V carrier without the PCI link queryHEADmain

Current Windows PF drivers do not emulate the E610 link-status query at
PCI configuration offset 0x209.  The installed Intel ixw 1.8.54.0 driver
returns zero even with the physical port connected, leaving the VF at
no carrier after an otherwise successful hn handoff.

When the query returns zero, use VFLINKS for carrier only.  Its speed
field is not the negotiated E610 line rate: the tested 1 Gb/s port
reports the default 10 Gb/s encoding.  Publish an unknown speed instead,
including in the verbose link-up diagnostic, and document this fallback.
Keep nonzero PCI results authoritative and reject all-ones reads.  Other
Hyper-V families and the native E610 mailbox path are unchanged.

A zero result cannot distinguish an unsupported query from a down link;
in either case this fallback relies on the legacy carrier indication.
The connected and disconnected ports reported VFLINKS.UP set and clear,
respectively, while both returned zero from the PCI query.

MFC after:	2 weeks
Sponsored by:	BBOX.io

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