xen-all.c: fix multiply issue for int and uint types

xen-all.c: fix multiply issue for int and uint types

If the two multiply operands are int and uint types separately,
the int type will be transformed to uint firstly, which is not the
intent in our code piece. The fix is to add (int64_t) transform
for the uint type before the multiply.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>