vmbus: fix strncpy related warnings

The code that was manipulating interface names with ifreq was
causing warnings about possible truncation and non terminated
strings.

These are warnings only since kernel would allow a interface
name > 15 characters anyway.

Change-Id: I794a94fe310b8568403d4e3523c61d53468a6f02
Reported-by: Burt Silverman <burtms@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>