qemu_command: Resolve Coverity REVERSE_INULL

qemu_command: Resolve Coverity REVERSE_INULL

In qemuNetworkIfaceConnect() a call to virNetDevBandwidthSet() is
made where the function prototype requires the first parameter
(net->ifname) to be non NULL.  Coverity complains that the subsequent
non NULL check for net->ifname prior to the next call gets flagged as
an unnecessary check.  Resolve by removing the extra check