kvm_stat: Rework platform detection

kvm_stat: Rework platform detection

The current platform detection is a little bit messy. We look for lines
in /proc/cpuinfo starting with 'flags' OR 'vendor-id', and scan both
for values we know will only occur in one or the other. We also keep
scanning once we've found a value, which could be a feature, but isn't
in this case.

We'd also like to add another platform, powerpc, which will just make it
worse. So clean it up in preparation.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>