For versions of nettle < 3.0.0, the cipher functions took a
'void *ctx' and 'unsigned len' instad of 'const void *ctx'
and 'size_t len'. The xts functions though are builtin to
QEMU and always expect the latter signatures. Define a
second set of wrappers to use with the correct signatures
needed by XTS mode.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>