vhost: clear out unused SCM_RIGHTS file descriptors

vhost: clear out unused SCM_RIGHTS file descriptors

The number of file descriptors received is not stored by vhost_user.c.
vhost_user_set_mem_table() assumes that memory.nregions matches the
number of file descriptors received, but nothing guarantees this:

  for (i = 0; i < memory.nregions; i++)
      close(pmsg->fds[i]);

Another questionable code snippet is:

  case VHOST_USER_SET_LOG_FD:
      close(msg.fds[0]);

If not enough file descriptors were received then fds[] contains
uninitialized data from the stack (see read_fd_message()).  This might
cause non-vhost file descriptors to be closed if the uninitialized data
happens to match.

Refactoring vhost_user.c to pass around and check the number of file
descriptors everywhere would make the code more complex.  It is simpler
for read_fd_message() to set unused elements in fds[] to -1.  This way
close(-1) is called and no harm is done.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>