KVM: PPC: Book3S HV: Add 'online' register to ONE_REG interface

This adds a new KVM_REG_PPC_ONLINE register which userspace can set
to 0 or 1 via the GET/SET_ONE_REG interface to indicate whether it
considers the VCPU to be offline (0), that is, not currently running,
or online (1).  This will be used in a later patch to configure the
register which controls PURR and SPURR accumulation.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>