[mythtv] mythvideo DB enhancement

[George Nassas]

One other thing I keep forgetting about - what should be done about  
coverfiles? They're currently stored with absolute pathnames and  
really ought to be converted to relative paths like videos but it  
sounds dumb to have a storage group just for covers.

You'd almost want a binary blob column in videometadata for covers.  
On my system the covers take up 13MB while mythconverg is 110MB so  
it's roughly a 10% increase.

Or, they could be moved alongside the videos the way it's done with  
recordings and preview images.

I'm OK with any of the three options but what do you all think?

- George