scritps/runqemu: Ensure we only have two serial ports

scritps/runqemu: Ensure we only have two serial ports

I have a theory that some of the console boot issues we're seeing are due to
starting images with three serial ports yet only starting gettys on two of them.

This means that occasionally, depending on the port numbering we may not get
a login prompt on the console we expect it on.

To fix this, change the runqemu code so that if serial ports are passed in on
the commandline (as is the case in automated testing), we don't add any other
GUI serial consoles.

We do need to make sure we do have at least two serial ports since we don't want
getty timeout warnings.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>