net: merge virNetworkGetDHCPLeases and virNetworkGetDHCPLeasesForMAC
    
    Instead of maintaining two very similar APIs, add the "@mac" parameter
    to virNetworkGetDHCPLeases and kill virNetworkGetDHCPLeasesForMAC. Both
    of those functions would return data the same way, so making @mac an
    optional filter simplifies a lot of stuff.