"virtual" C++ keyword used in rte_devargs.h and causes compilation error in C++

[Ming Zhao <mzhao-82A4bShDkBpJQxFrfMEhiVaTQe2KTcn/@public.gmane.org>]

The code is in rte_devargs.h:

rte_devargs.h:

                /** Used if type is RTE_DEVTYPE_VIRTUAL. */
                struct {
                        /** Driver name. */
                        char drv_name[32];
                } virtual;
        };

Which caused clang compiler to report error when this file is included
by a cpp file, the error message is:

In file included from net/dpdk/testing/base-test.cc:3:
In file included from net/dpdk/testing/base-test.h:8:
third-party/dpdk/lib/librte_eal/common/include/rte_devargs.h:89:5:
error: 'virtual' can only appear on non-static
      member functions
                } virtual;
                  ^

I think we should try to pick another name for this field. I would
suggest calling it "vdev" instead, or I'll be happy to take another name
if someone comes with a different idea.

Thanks!
Ming