vhost: add missing C++ guards

vhost: add missing C++ guards

Some public header files were missing 'extern "C"' C++ guards,
and couldn't be used by C++ applications. Add the missing guards.

Fixes: d7280c9fffcb ("vhost: support selective datapath")
Fixes: 78639d54563a ("vhost: introduce async enqueue registration API")
Fixes: 3bb595ecd682 ("vhost/crypto: add request handler")
Fixes: 94c16e89d779 ("vhost: mark vDPA driver API as internal")
Cc: stable@dpdk.org
Signed-off-by: Brian Dooley <brian.dooley@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>