xenvirt: convert to use of PMD_REGISTER_DRIVER and fix linking

xenvirt: convert to use of PMD_REGISTER_DRIVER and fix linking

Convert the xenvirt driver to use the PMD_REGISTER_DRIVER macro.
This means that the test applications now have no reference to the xenvirt library
when building DSO's and must specify its use on the command line with the -d
option.  Static linking will still initalize the driver automatically.

A few notes:

xenvirt was unbuildable as of commit 4c39baf297d10c217e7d3e7370f26a1fede58308..
That commit neglected to include the rte_vdev.h header, so several structs were
left undefined.  This patch includes a fix for that as well.

Also, The linkage for xenvirt is broken in much the same way pmd_ring was, in
that the xenvirt pmd has a function that is called directly from applications
(the example being the testpmd application).  The function is
rte_mempool_gntalloc_create, and should clearly be moved into the rte_mempool
library, with the supporting code in the function implementation moved to a new
xenvirt library separate from the pmd.  This is a large undertaking that
detracts from the purpose of this series however, and so for now, I'm leaving
the linkage to the application in place, and will address this issue in a later
series

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>