Enable NVMe start controller for Windows guest.

Enable NVMe start controller for Windows guest.

Windows seems to send two separate calls to NVMe controller configuration. The
first sends configuration info and the second the enable bit. I couldn't
enable the Windows 8.1 in-box NVMe driver with base Qemu. I made the
following change to store the configuration data and then handle enable and
NVMe driver works on Windows 8.1.

I am not a Windows expert and I'm not entirely sure this is the correct
approach. I'm offering it for anyone who wishes to use NVMe on Windows 8.1
using Qemu.

I have tested this change with Linux and Windows guests with NVMe devices.

Signed-off-by: Daniel Stekloff <dan@wendan.org>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>