machine: Move nvdimms state into struct MachineState

machine: Move nvdimms state into struct MachineState

As NVDIMM support is looming for ARM and SPAPR, let's
move the acpi_nvdimm_state to the generic machine struct
instead of duplicating the same code in several machines.
It is also renamed into nvdimms_state and becomes a pointer.

nvdimm and nvdimm-persistence become generic machine options.
They become guarded by a nvdimm_supported machine class member.
We also add a description for those options.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Suggested-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190308182053.5487-3-eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>