net/virtio-user: fix build with icc

net/virtio-user: fix build with icc

Implicit int to enum conversion is not allowed when icc is used as
the compiler. It raises the compiling error like,
    drivers/net/virtio/virtio_user/vhost_user.c(257):
        error #188: enumerated type mixed with another type
    msg.request = req;
                ^

The fix is simple, change the type of parameter req to enum
vhost_user_request.

Fixes: 6a84c37e3975 ("net/virtio-user: add vhost-user adapter layer")
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>