net: don't set native endianness

commit 5be7d9f1
    vhost-net: tell tap backend about the vnet endianness
makes vhost net always try to set LE - even if that matches the
native endian-ness.

This makes it fail on older kernels on x86 without TUNSETVNETLE support.

To fix, make qemu_set_vnet_le/qemu_set_vnet_be skip the
ioctl if it matches the host endian-ness.

Reported-by: Marcel Apfelbaum <marcel@redhat.com>
Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
(cherry picked from commit 052bd52f)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>