tools/virtiofsd: Check vu_init() return value (CID 1435958)

Since commit 6f5fd837, vu_init() can fail if malloc() returns NULL.

This fixes the following Coverity warning:

  CID 1435958 (#1 of 1): Unchecked return value (CHECKED_RETURN)

Fixes: 6f5fd837 ("libvhost-user: support many virtqueues")
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201102092339.2034297-1-philmd@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>