kvm: stats: tell userspace which values are boolean

Some of the statistics values exported by KVM are always only 0 or 1.
It can be useful to export this fact to userspace so that it can track
them specially (for example by polling the value every now and then to
compute a % of time spent in a specific state).

Therefore, add "boolean value" as a new "unit".  While it is not exactly
a unit, it walks and quacks like one.  In particular, using the type
would be wrong because boolean values could be instantaneous or peak
values (e.g. "is the rmap allocated?") or even two-bucket histograms
(e.g. "number of posted vs. non-posted interrupt injections").

Suggested-by: Amneesh Singh <natto@weirdnatto.in>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/uapi/linux/kvm.h

@@ -2083,6 +2083,7 @@ struct kvm_stats_header {
 #define KVM_STATS_UNIT_BYTES		(0x1 << KVM_STATS_UNIT_SHIFT)
 #define KVM_STATS_UNIT_SECONDS		(0x2 << KVM_STATS_UNIT_SHIFT)
 #define KVM_STATS_UNIT_CYCLES		(0x3 << KVM_STATS_UNIT_SHIFT)
+#define KVM_STATS_UNIT_BOOLEAN		(0x4 << KVM_STATS_UNIT_SHIFT)
 #define KVM_STATS_UNIT_MAX		KVM_STATS_UNIT_CYCLES
 
 #define KVM_STATS_BASE_SHIFT		8