Stop using RtlStringCbPrintfA()

Stop using RtlStringCbPrintfA()

It appears that linking an NDIS driver with any ntstrsafe function (when
building with VS2012 and the 8.0 WDK at least) has the side effect of
requiring use of DbgPrint(). Whilst this may not sound like a problem, it
is. This is because the NDISTest 6.0 1c_KernelCalls test insists that any
use of DbgPrint is illegal and so it's impossible to logo an NDIS driver
using ntstrsafe functions.

This patch works around the issue by dynamically linking (at run-time) to
the ntdll _snprintf_s function as a replacement for calls to
RtlStringCbPrintfA(). This entails importing the dynamic linking code from
XENVIF since MmGetSystemRoutineAddress() doesn't seem to work for that
symbol.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>