virtio-devices: 8 MiB-aligned initial BAR placement

Windows 11 PnP rebalance rewrites peer BARs into the same range CH
packed the initial layout at, causing move_bar() failures and boot
deadlock. Pack Mmio64 BARs at 8 MiB stride. Mmio32 isn't wide enough
for the same stride, but its BARs don't participate in guest BAR
rebalancing.

On restore, pin the BAR to the snapshot address (alignment=None) so a
guest-relocated BAR with smaller alignment is accepted.

Signed-off-by: CMGS <ilskdw@gmail.com>

virtio-devices/src/transport/pci_device.rs

@@ -281,6 +281,8 @@ const MSIX_PBA_BAR_OFFSET: u64 = next_bar_addr(MSIX_TABLE_BAR_OFFSET, MSIX_TABLE
 const MSIX_PBA_SIZE: u64 = 0x800;
 // The BAR size must be a power of 2.
 const CAPABILITY_BAR_SIZE: u64 = (MSIX_PBA_BAR_OFFSET + MSIX_PBA_SIZE).next_power_of_two();
+// Align larger than natural alignment to work around Windows driver issues
+const VIRTIO_PCI_BAR_ALIGN: u64 = 0x80_0000;
 const VIRTIO_COMMON_BAR_INDEX: u8 = 0;
 const VIRTIO_SHM_BAR_INDEX: usize = 2;
 
@@ -1044,12 +1046,13 @@ impl PciDevice for VirtioPciDevice {
         // See http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-740004
         let (virtio_pci_bar_addr, region_type) = if use_64bit_bar {
             let region_type = PciBarRegionType::Memory64BitRegion;
+            let alignment = if restoring {
+                None
+            } else {
+                Some(VIRTIO_PCI_BAR_ALIGN)
+            };
             let addr = mmio64_allocator
-                .allocate(
-                    settings_bar_addr,
-                    CAPABILITY_BAR_SIZE,
-                    Some(CAPABILITY_BAR_SIZE),
-                )
+                .allocate(settings_bar_addr, CAPABILITY_BAR_SIZE, alignment)
                 .ok_or(PciDeviceError::IoAllocationFailed(CAPABILITY_BAR_SIZE))?;
             (addr, region_type)
         } else {