virNetDevGetEthtoolGFeatures: Avoid use of VIR_ALLOC_VAR

virNetDevGetEthtoolGFeatures: Avoid use of VIR_ALLOC_VAR

In this case we need a 'struct ethtool_gfeatures' followed by two
'struct ethtool_get_features_block' so there's no risk of overflow.

Use g_malloc0 and sizeof() to allocate the memory instead of
VIR_ALLOC_VAR.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>