KVM: PPC: Book3S: Move KVM_REG_PPC_WORT to an unused register number

Commit b005255e ("KVM: PPC: Book3S HV: Context-switch new POWER8
SPRs") added a definition of KVM_REG_PPC_WORT with the same register
number as the existing KVM_REG_PPC_VRSAVE (though in fact the
definitions are not identical because of the different register sizes.)

For clarity, this moves KVM_REG_PPC_WORT to the next unused number,
and also adds it to api.txt.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>