Use g_strdup where VIR_STRDUP's return value was propagated

Use g_strdup where VIR_STRDUP's return value was propagated

All the callers of these functions only check for a negative
return value.

However, virNetDevOpenvswitchGetVhostuserIfname is documented
as returning 1 for openvswitch interfaces so preserve that.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>