jtag_vpi: ensured constant packet size & endianness

jtag_vpi: ensured constant packet size & endianness

Made sure that size and endianness of jtag_vpi structures sent
over the network is the same, regardless of the platform.

Little endian chosen to maintain as much compatibility
with existing OpenOCD builds as possible.

Matching change in the original jtag_vpi server:
https://github.com/fjullien/jtag_vpi/pull/4

Change-Id: Ib839fea9bb2d5190b5643c970b89333b286dce71
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/5152
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins