hw/nvme: add atomic write support

hw/nvme: add atomic write support

Adds support for the controller atomic parameters: AWUN and AWUPF. Atomic
Compare and Write Unit (ACWU) is not currently supported.

Writes that adhere to the ACWU and AWUPF parameters are guaranteed to be atomic.

New NVMe QEMU Parameters (See NVMe Specification for details):
       atomic.dn (default off) - Set the value of Disable Normal.
       atomic.awun=UINT16 (default: 0)
       atomic.awupf=UINT16 (default: 0)

By default (Disable Normal set to zero), the maximum atomic write size is
set to the AWUN value.  If Disable Normal is set, the maximum atomic write
size is set to AWUPF.

Signed-off-by: Alan Adamson <alan.adamson@oracle.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>