conf: Drop restrictions on rng backend path
    
    Currently we only allow /dev/random and /dev/hwrng as host input
    for <rng><backend model='random'/> device. This was added after
    various upstream discussions in commit 4932ef45
    
    However this restriction has generated quite a few complaints over
    the years, so a new discussion was initiated:
    
    http://www.redhat.com/archives/libvir-list/2016-April/msg00987.html
    
    Several people suggested removing the restriction, and nobody really
    spoke up to defend it. So this patch drops the path restriction
    entirely
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1074464