pc: piix4_pm: init legacy PCI hotplug when running on Xen

pc: piix4_pm: init legacy PCI hotplug when running on Xen

If user starts QEMU with "-machine pc,accel=xen", then
compat property in xenfv won't work and it would cause error:
"Unsupported bus. Bus doesn't have property 'acpi-pcihp-bsel' set"
when PCI device is added with -device on QEMU CLI.

From: Igor Mammedov <imammedo@redhat.com>

In case of Xen instead of using compat property, just use the fact
that xen doesn't use QEMU's fw_cfg/acpi tables to switch piix4_pm
into legacy PCI hotplug mode when Xen is enabled.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Li Liang <liang.z.li@intel.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>