xl: fix broken xl vcpu-list output (tool hangs on large machines)

xl: fix broken xl vcpu-list output (tool hangs on large machines)

The algorithm for printing the CPU affinity in a condensed way
looks for a set bit in a zero-byte:
             for (i = 0; !(pcpumap & 1); ++i, pcpumap >>= 1)
Looking at the code I found that it is entirely broken if more than 8
CPUs are used. Beside that endless loop issue the output is totally
bogus except for the "any CPU" case, which is handled explicitly earlier.
I tried to fix it, but the whole approach does not work if the outer
loops actually iterates (executing more than once).
This fix reimplements the whole algorithm in a clean (though not much
optimized way). It survived some unit-testing.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>