libxl: fix JSON generator for uint64_t

libxl: fix JSON generator for uint64_t

yajl_gen_integer cannot cope with uint64_t, because it takes a signed
long long. If we pass to it an uint64_t number which is between INT_MAX
and UINT_MAX, it generates a negative number. Later when we feed this
generated number into parser, the result gets signed extended, which is
wrong.

A new function called libxl__uint64_gen_json is introduced to handle
uint64_t. It utilises yajl_gen_number to generate numbers.

Also removed a duplicated definition of MemKB while I was there.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>