net/virtio-user: fix inconsistent name

net/virtio-user: fix inconsistent name

The commit cb6696d22023 ("drivers: update registration macro usage")
changes the name from virtio-user to virtio_user, because hyphen
cannot be used in a C symbol name. However, this commit does not
update the strings in docs and source code, which could lead to
failure to start this device as per the docs.

This patch updates related strings in the docs and source code.

Fixes: cb6696d22023 ("drivers: update registration macro usage")
Reported-by: Tiwei Bie <tiwei.bie@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>