kvm/x86 : add document for coalesced mmio

This copies the vcpu's kvm_nested_state struct from userspace to the kernel.  For

the definition of struct kvm_nested_state, see KVM_GET_NESTED_STATE.

4.116 KVM_(UN)REGISTER_COALESCED_MMIO

Capability: KVM_CAP_COALESCED_MMIO

Architectures: all

Type: vm ioctl

Parameters: struct kvm_coalesced_mmio_zone

Returns: 0 on success, < 0 on error

Coalesced mmio is a performance optimization that defers hardware

register write emulation so that userspace exits are avoided.  It is

typically used to reduce the overhead of emulating frequently accessed

hardware registers.

When a hardware register is configured for coalesced mmio, write accesses

do not exit to userspace and their value is recorded in a ring buffer

that is shared between kernel and userspace.

Coalesced mmio is used if one or more write accesses to a hardware

register can be deferred until a read or a write to another hardware

register on the same device.  This last access will cause a vmexit and

userspace will process accesses from the ring buffer before emulating

it. That will avoid exiting to userspace on repeated writes to the

first register.

5. The kvm_run structure

------------------------