netdev: silence valgrind warning about ioctl use
    
    Valgrind complained:
    
    ==3770== Syscall param ioctl(SIOCETHTOOL) points to uninitialised byte(s)
    ==3770==    at 0x919D407: ioctl (syscall-template.S:81)
    ==3770==    by 0x530FE7E: rpl_ioctl (ioctl.c:42)
    ==3770==    by 0x50CB433: virNetDevFeatureAvailable (virnetdev.c:2764)
    ==3770==    by 0x50CB6A7: virNetDevGetFeatures (virnetdev.c:2830)
    ==3770==    by 0x1F0E5347: udevProcessNetworkInterface (node_device_udev.c:722)
    ==3770==    by 0x1F0E689F: udevGetDeviceDetails (node_device_udev.c:1300)
    ==3770==    by 0x1F0E6E06: udevAddOneDevice (node_device_udev.c:1422)
    ==3770==    by 0x1F0E6FB8: udevProcessDeviceListEntry (node_device_udev.c:1464)
    ==3770==    by 0x1F0E70CF: udevEnumerateDevices (node_device_udev.c:1494)
    ==3770==    by 0x1F0E7BB4: nodeStateInitialize (node_device_udev.c:1806)
    ==3770==    by 0x51B4303: virStateInitialize (libvirt.c:777)
    ==3770==    by 0x11DEE7: daemonRunStateInit (libvirtd.c:906)
    ==3770==  Address 0x228e38d4 is on thread 12's stack
    ==3770==  in frame #2, created by virNetDevFeatureAvailable (virnetdev.c:2750)
    
    * src/util/virnetdev.c (virNetDevFeatureAvailable): Initialize all
    bytes of ifr.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>