virtio-pci: Changed return values for "notify", "device" and "isr" read.

At some point, after unplugging virtio-pci the virtio device may be unrealised,
but the memory regions may be present in flatview. So, it's a possible situation
when memory region's callbacks are called for "unplugged" device.

Previous two patches made sure this case does not cause QEMU to crash.
This patch adds check for "notify" memory region. Now reads will return "-1" if a virtio
device is not present on a virtio bus.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1938042
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1743098

Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Message-Id: <20210609095843.141378-4-andrew@daynix.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -1349,6 +1349,11 @@ static void virtio_pci_common_write(void *opaque, hwaddr addr,
 static uint64_t virtio_pci_notify_read(void *opaque, hwaddr addr,
                                        unsigned size)
 {
+    VirtIOPCIProxy *proxy = opaque;
+    if (virtio_bus_get_device(&proxy->bus) == NULL) {
+        return UINT64_MAX;
+    }
+
     return 0;
 }
 
@@ -1386,7 +1391,7 @@ static uint64_t virtio_pci_isr_read(void *opaque, hwaddr addr,
     uint64_t val;
 
     if (vdev == NULL) {
-        return 0;
+        return UINT64_MAX;
     }
 
     val = qatomic_xchg(&vdev->isr, 0);
@@ -1407,7 +1412,7 @@ static uint64_t virtio_pci_device_read(void *opaque, hwaddr addr,
     uint64_t val;
 
     if (vdev == NULL) {
-        return 0;
+        return UINT64_MAX;
     }
 
     switch (size) {