Drivers: hv: vmbus: Initialize request offers message for Isolation VM

Initialize memory of request offers message to be sent to the host so
padding or uninitialized fields do not leak guest memory contents.

Signed-off-by: Juan Vazquez <juvazq@linux.microsoft.com>
Link: https://lore.kernel.org/r/20220105192746.23046-1-juvazq@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>