app/testpmd: fix running without Xen whereas enabled

app/testpmd: fix running without Xen whereas enabled

With LIBRTE_PMD_XENVIRT enabled testpmd is built in a way to ONLY work
in XEN environments.
It will surface as:
   PMD: gntalloc: ioctl error
   EAL: Error - exiting with code: 1
     Cause: Creation of mbuf pool for socket 0 failed

With LIBRTE_PMD_XENVIRT enabled this now tries the xen style grant
table allocation, but falls back gracefully for the normal allocation.

The only thing left in the log will be the
   PMD: gntalloc: ioctl error

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>