virDomainRedirdevDef: Introduce find & remove routines
    
    Basically, there are just two functions introduced here:
    virDomainRedirdevDefFind which looks up given redirdev in domain
    definition, and virDomainRedirdevDefRemove which removes the
    device at given index in the array of devices.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>