nvme: introduce PMR support from NVMe 1.4 spec

nvme: introduce PMR support from NVMe 1.4 spec

This patch introduces support for PMR that has been defined as part of NVMe 1.4
spec. User can now specify a pmrdev option that should point to HostMemoryBackend.
pmrdev memory region will subsequently be exposed as PCI BAR 2 in emulated NVMe
device. Guest OS can perform mmio read and writes to the PMR region that will stay
persistent across system reboot.

Signed-off-by: Andrzej Jakowski <[email protected]>
Reviewed-by: Klaus Jensen <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20200330164656[email protected]>
Reviewed-by: Keith Busch <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>