NVMe: Initial commit for new storage interface

NVMe: Initial commit for new storage interface

Initial commit for emulated Non-Volatile-Memory Express (NVMe) pci
storage device.

NVMe is an open, industry driven storage specification defining
an optimized register and command set designed to deliver the full
capabilities of non-volatile memory on PCIe SSDs. Further information
may be found on the organizations website at:

http://www.nvmexpress.org/

This commit implements the minimum from the specification to work with
existing drivers.

Cc: Keith Busch <[email protected]>
Signed-off-by: Keith Busch <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>