vhost-vdpa: return -EFAULT on copy_to_user() failure

The copy_to_user() function returns the number of bytes remaining to be
copied.  However, we need to return a negative error code, -EFAULT, to
the user.

Fixes: 87f4c217413a ("vhost-vdpa: introduce uAPI to get the number of virtqueue groups")
Fixes: e96ef636f154 ("vhost-vdpa: introduce uAPI to get the number of address spaces")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Message-Id: <YotG1vXKXXSayr63@kili>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>