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>