kvmtool: Copy some bits from Linux' byteorder.h

Instead of moving the whole include file in, we just copy the needed
definitions from byteorder/generic.h into the kvmtool headers.
This avoids needless clutter of the include directory.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>