KVM: stats: Fix value for KVM_STATS_UNIT_MAX for boolean stats

commit 1b870fa5 ("kvm: stats: tell userspace which values are
boolean") added a new stat unit (boolean) but failed to raise
KVM_STATS_UNIT_MAX.

Fix by pointing UNIT_MAX at the new max value of UNIT_BOOLEAN.

Fixes: 1b870fa5

 ("kvm: stats: tell userspace which values are boolean")
Reported-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20220719125229.2934273-1-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>