Drivers: hv: balloon: Initialize the transaction ID just before sending the packet

Each message sent from the guest carries with it a transaction ID.
Assign the transaction ID just before putting the message on the VMBUS.
This would help in debugging on the host side.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>