arm/acpi: fix an out of spec _UID for PCI root

On ARM/virt machine type QEMU currently reports an incorrect _UID in
ACPI.

The particular node in question is the primary PciRoot (PCI0 in ACPI),
which gets assigned PCI0 in ACPI UID and 0 in the
DevicePath. This is due to the _UID assigned to it by build_dsdt in
hw/arm/virt-acpi-build.c Which does not correspond to the primary PCI
identifier given by pcibus_num in hw/pci/pci.c

In UEFI v2.8, section "10.4.2 Rules with ACPI _HID and _UID" ends with
the paragraph,

    Root PCI bridges will use the plug and play ID of PNP0A03, This will
    be stored in the ACPI Device Path _HID field, or in the Expanded
    ACPI Device Path _CID field to match the ACPI name space. The _UID
    in the ACPI Device Path structure must match the _UID in the ACPI
    name space.

(See especially the last sentence.)

A similar bug has been reported on i386, on that architecture it has
been reported to confuse at least macOS which uses ACPI UIDs to build
the DevicePath for NVRAM boot options, while OVMF firmware gets them via
an internal channel through QEMU.  When UEFI firmware and ACPI have
different values, this makes the underlying operating system unable to
report its boot option.

Cc: qemu-stable@nongnu.org
Reported-by: Vitaly Cheptsov <vit9696@protonmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

hw/arm/virt-acpi-build.c

@@ -170,7 +170,7 @@ static void acpi_dsdt_add_pci(Aml *scope, const MemMapEntry *memmap,
     aml_append(dev, aml_name_decl("_CID", aml_string("PNP0A03")));
     aml_append(dev, aml_name_decl("_SEG", aml_int(0)));
     aml_append(dev, aml_name_decl("_BBN", aml_int(0)));
-    aml_append(dev, aml_name_decl("_UID", aml_string("PCI0")));
+    aml_append(dev, aml_name_decl("_UID", aml_int(0)));
     aml_append(dev, aml_name_decl("_STR", aml_unicode("PCIe 0 Device")));
     aml_append(dev, aml_name_decl("_CCA", aml_int(1)));