Wire up the building of the virtiofsd in tools.

virtiofsd relies on Linux-specific system calls and seccomp.  Anyone
wishing to port it to other host operating systems should do so
carefully and without reducing security.

Only allow building on Linux hosts.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>