Ensure nodeinfo struct is initialized to zero

Ensure nodeinfo struct is initialized to zero

When linuxNodeInfoCPUPopulate() method triggered use of an
uninitialize value, since it did not initialize the 'sockets'
field in the virNodeInfoPtr struct:

==30020== Conditional jump or move depends on uninitialised value(s)
==30020==    at 0x5125DBD: linuxNodeInfoCPUPopulate (nodeinfo.c:513)
==30020==    by 0x51261A0: nodeGetInfo (nodeinfo.c:884)
==30020==    by 0x149B9B10: qemuCapsInit (qemu_capabilities.c:846)
==30020==    by 0x14A11B25: qemuCreateCapabilities (qemu_driver.c:424)
==30020==    by 0x14A12426: qemuStartup (qemu_driver.c:874)
==30020==    by 0x512A7AF: virStateInitialize (libvirt.c:822)
==30020==    by 0x40DE04: daemonRunStateInit (libvirtd.c:877)
==30020==    by 0x50ADCE5: virThreadHelper (virthreadpthread.c:161)
==30020==    by 0x328CA07D14: start_thread (pthread_create.c:308)
==30020==    by 0x328C6F246C: clone (clone.S:114)
(happened twice)

        if (socks > nodeinfo->sockets)    <--- here
            nodeinfo->sockets = socks;

Rather than doing this for each field, just make the caller memset
the entire struct to zero.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>