hyperv/kvp: Don't mix message status codes and function return values.

While I'm here, move message status codes to hv_utilreg.h, since they
will be used by the upcoming VSS stuffs.

Submitted by: Hongjiang Zhang <honzhan microsoft com>
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8391