eal: fix C++ build

eal: fix C++ build

'virtual' is a keyword and can't be used if the code is to compile with
C++ compilers.

If rte_devargs.h was included in C++ code, compilation with clang++
failed with an error. g++ did not fail, but only because of a bug
that treats it as an anonymous struct with a decl-specifier which it
ignores.

This simply renames the member to 'virt'.

Reported-by: Ming Zhao <mzhao@luminatewireless.com>
Signed-off-by: Christoph Gysin <christoph.gysin@gmail.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
Acked-by: David Marchand <david.marchand@6wind.com>