libxl: fix build with glibc < 2.9

libxl: fix build with glibc < 2.9

htobe*() and be*toh() don't exist there. While replacing the 32-bit
ones with hton() and ntoh() would be possible, there wouldn't be an
obvious replacement for the 64-bit ones. Hence just take what current
glibc (2.21) has (assuming __bswap_*() exists, which it does back to
at least 2.4 according to my checking).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>