examples/vhost: fix ioat dependency

examples/vhost: fix ioat dependency

Fix vhost-switch compiling issue when ioat dependency is missing.
Change 'RTE_x86' check into 'RTE_RAW_IOAT' check in meson build file.
Use 'RTE_RAW_IOAT' to control conditional compiling in source file.
Clean some codes.

Fixes: abec60e7115d ("examples/vhost: support vhost async data path")
Fixes: 3a04ecb21420 ("examples/vhost: add async vhost args parsing")
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Cheng Jiang <cheng1.jiang@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: David Marchand <david.marchand@redhat.com>