hw/net/vmxnet3: Fix code to work on big endian hosts, too

Since commit ab06ec43 we test the vmxnet3 device in the
pxe-tester, too (when running "make check SPEED=slow"). This now
revealed that the code is not working there if the host is a big
endian machine (for example ppc64 or s390x) - "make check SPEED=slow"
is now failing on such hosts.

The vmxnet3 code lacks endianness conversions in a couple of places.
Interestingly, the bitfields in the structs in vmxnet3.h already tried to
take care of the *bit* endianness of the C compilers - but the code missed
to change the *byte* endianness when reading or writing the corresponding
structs. So the bitfields are now wrapped into unions which allow to change
the byte endianness during runtime with the non-bitfield member of the union.
With these changes, "make check SPEED=slow" now properly works on big endian
hosts, too.

Reported-by: David Gibson <dgibson@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: David Gibson <dgibson@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>