Implement RPC calls for virNetworkGetDHCPLeases, virNetworkGetDHCPLeasesForMAC

daemon/remote.c
   * Define remoteSerializeNetworkDHCPLeases,
            remoteDispatchNetworkGetDHCPLeases
   * Define remoteDispatchNetworkGetDHCPLeasesForMAC
   * Define helper function remoteSerializeDHCPLease

src/remote/remote_driver.c
   * Define remoteNetworkGetDHCPLeases
   * Define remoteNetworkGetDHCPLeasesForMAC
   * Define helper function remoteSerializeDHCPLease

src/remote/remote_protocol.x
   * New RPC procedure: REMOTE_PROC_NETWORK_GET_DHCP_LEASES
   * Define structs remote_network_dhcp_leases, remote_network_get_dhcp_leases_args,
                    remote_network_get_dhcp_leases_ret
   * New RPC procedure: REMOTE_PROC_NETWORK_GET_DHCP_LEASES_FOR_MAC
   * Define structs remote_network_dhcp_leases_for_mac, remote_network_get_dhcp_leases_for_mac_args,
                    remote_network_get_dhcp_leases_for_mac_ret

src/remote_protocol-structs
   * New structs added

src/rpc/gendispatch.pl
   * Add exception (s/Dhcp/DHCP) for auto-generating names of the remote functions
     in daemon/remote_dispatch.h