nvdimm, acpi: support NFIT platform capabilities

Add a machine command line option to allow the user to control the Platform
Capabilities Structure in the virtualized NFIT.  This Platform Capabilities
Structure was added in ACPI 6.2 Errata A.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/nvdimm.txt

@@ -153,3 +153,30 @@ guest NVDIMM region mapping structure.  This unarmed flag indicates
 guest software that this vNVDIMM device contains a region that cannot
 accept persistent writes. In result, for example, the guest Linux
 NVDIMM driver, marks such vNVDIMM device as read-only.
+
+Platform Capabilities
+---------------------
+
+ACPI 6.2 Errata A added support for a new Platform Capabilities Structure
+which allows the platform to communicate what features it supports related to
+NVDIMM data durability.  Users can provide a capabilities value to a guest via
+the optional "nvdimm-cap" machine command line option:
+
+    -machine pc,accel=kvm,nvdimm,nvdimm-cap=2
+
+This "nvdimm-cap" field is an integer, and is the combined value of the
+various capability bits defined in table 5-137 of the ACPI 6.2 Errata A spec.
+
+Here is a quick summary of the three bits that are defined as of that spec:
+
+Bit[0] - CPU Cache Flush to NVDIMM Durability on Power Loss Capable.
+Bit[1] - Memory Controller Flush to NVDIMM Durability on Power Loss Capable.
+         Note: If bit 0 is set to 1 then this bit shall be set to 1 as well.
+Bit[2] - Byte Addressable Persistent Memory Hardware Mirroring Capable.
+
+So, a "nvdimm-cap" value of 2 would mean that the platform supports Memory
+Controller Flush on Power Loss, a value of 3 would mean that the platform
+supports CPU Cache Flush and Memory Controller Flush on Power Loss, etc.
+
+For a complete list of the flags available and for more detailed descriptions,
+please consult the ACPI spec.