KVM: x86: do not allow re-enabling quirks

Allowing arbitrary re-enabling of quirks puts a limit on what the
quirks themselves can do, since you cannot assume that the quirk
prevents a particular state.  More important, it also prevents
KVM from disabling a quirk at VM creation time, because userspace
can always go back and re-enable that.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>