Documentation: kvm: Fix mention to number of ioctls classes

In api.txt it is said that KVM ioctls belong to three classes
but in reality it is four. Fixed this, but do not count categories
anymore to avoid such as outdated information in the future.
Signed-off-by: NWainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.txt

@@ -5,7 +5,7 @@ The Definitive KVM (Kernel-based Virtual Machine) API Documentation
 ----------------------
 
 The kvm API is a set of ioctls that are issued to control various aspects
-of a virtual machine.  The ioctls belong to three classes:
+of a virtual machine.  The ioctls belong to the following classes:
 
  - System ioctls: These query and set global attributes which affect the
    whole kvm subsystem.  In addition a system ioctl is used to create