hv_utils: Add comment about max VMbus packet size in VSS driver

The VSS driver allocates a VMbus receive buffer significantly
larger than sizeof(hv_vss_msg), with no explanation. To help
prevent future mistakes, add a #define and comment about why
this is done.

No functional change.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1644423070-75125-1-git-send-email-mikelley@microsoft.com


Signed-off-by: Wei Liu <wei.liu@kernel.org>