Documentation/kvm : Add documentation on Hypercalls and features used for PV spinlock

KVM_HC_KICK_CPU  hypercall added to wakeup halted vcpu in paravirtual spinlock
enabled guest.

KVM_FEATURE_PV_UNHALT enables guest to check whether pv spinlock can be enabled
in guest.

Thanks Vatsa for rewriting KVM_HC_KICK_CPU
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Gleb Natapov <gleb@redhat.com>