libvhost-user: fix clang enum-conversion warning

libvhost-user: fix clang enum-conversion warning

Now that the VhostUserMsg.request field is used for both master &
slave requests, since commit d84599f56c820d8c1ac9928a76500dcdfbbf194d:

contrib/libvhost-user/libvhost-user.c:953:20: error: implicit conversion from enumeration type 'enum VhostUserSlaveRequest' to different enumeration type 'VhostUserRequest' (aka 'enum VhostUserRequest') [-Werror,-Wenum-conversion]
        .request = VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG,
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190308140454.32437-2-marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>