nvme-pci: clear shadow doorbell memory on resets

The host memory doorbell and event buffers need to be initialized on
each reset so the driver doesn't observe stale values from the previous
instantiation.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Tested-by: John Levon <john.levon@nutanix.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>