sys-utils/xen-tools: remove usage of signed bitfields

When using a single-bit signed bitfield clang complains with:

xenalyze.c:6157:17: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1

Backport fix from upstream Xen.

Sponsored by: Citrix System R&D
Approved by: bapt (implicit)