OvmfPkg/PvScsiDxe: Reset device on ExitBootServices()

This causes the device to forget about the request/completion rings.
We allocated said rings in EfiBootServicesData type memory, and code
executing after ExitBootServices() is permitted to overwrite it.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2567
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Message-Id: <20200328200100.60786-17-liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>

OvmfPkg/PvScsiDxe/PvScsi.h

@@ -51,6 +51,7 @@ typedef struct {
 typedef struct {
   UINT32                          Signature;
   EFI_PCI_IO_PROTOCOL             *PciIo;
+  EFI_EVENT                       ExitBoot;
   UINT64                          OriginalPciAttributes;
   PVSCSI_RING_DESC                RingDesc;
   PVSCSI_DMA_BUFFER               *DmaBuf;