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anymore to avoid such as outdated information in the future.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: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>