virnetdev: fix build with old kernel

virnetdev: fix build with old kernel

Commit c9027d8f added a detection of NIC HW features, but some of them
are not available in old kernel.  Very old kernels lack enum
ethtool_flags and even if this enum is present, not all values are
available for all kernels.  To be sure that we have everything in kernel
that we need, we must check for existence of most of that flags, because
only few of them were defined at first.

Also to successfully build libvirt with older kernel we need to include
<linux/types.h> before <linux/ethtool.h> to have __u32 and friends
defined.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>