qemu: Allow the disk wwn to have "0x" prefix

qemu: Allow the disk wwn to have "0x" prefix

The recent qemu requires "0x" prefix for the disk wwn, this patch
changes virValidateWWN to allow the prefix, and prepend "0x" if
it's not specified. E.g.

qemu-kvm: -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,\
drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,wwn=6000c60016ea71ad:
Property 'scsi-hd.wwn' doesn't take value '6000c60016ea71ad'

Though it's a qemu regression, but it's nice to allow the prefix,
and doesn't hurt for us to always output "0x".