hw/acpi/ich9: Set ACPI PCI hot-plug as default on Q35

hw/acpi/ich9: Set ACPI PCI hot-plug as default on Q35

Q35 has three different types of PCI devices hot-plug: PCIe Native,
SHPC Native and ACPI hot-plug. This patch changes the default choice
for cold-plugged bridges from PCIe Native to ACPI Hot-plug with
ability to use SHPC and PCIe Native for hot-plugged bridges.

This is a list of the PCIe Native hot-plug issues that led to this
change:
    * no racy behavior during boot (see 110c477c2ed)
    * no delay during deleting - after the actual power off software
      must wait at least 1 second before indicating about it. This case
      is quite important for users, it even has its own bug:
          https://bugzilla.redhat.com/show_bug.cgi?id=1594168
    * no timer-based behavior - in addition to the previous example,
      the attention button has a 5-second waiting period, during which
      the operation can be canceled with a second press. While this
      looks fine for manual button control, automation will result in
      the need to queue or drop events, and the software receiving
      events in all sort of unspecified combinations of attention/power
      indicator states, which is racy and uppredictable.
    * fixes:
        * https://bugzilla.redhat.com/show_bug.cgi?id=1752465
        * https://bugzilla.redhat.com/show_bug.cgi?id=1690256

To return to PCIe Native hot-plug:
    -global ICH9-LPC.acpi-pci-hotplug-with-bridge-support=off

Known issue: older linux guests need the following flag
to allow hotplugged pci express devices to use io:
        -device pcie-root-port,io-reserve=4096.
io is unusual for pci express so this seems minor.
We'll fix this by a follow up patch.

Signed-off-by: Julia Suvorova <jusual@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20210713004205.775386-6-jusual@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>