acpi: avoid potential uninitialized access to cpu_hp_io_base

When building QEMU with Mingw64 toolchain I see a warning

 CC    x86_64-softmmu/hw/i386/acpi-build.o
  hw/i386/acpi-build.c: In function 'acpi_build':
  hw/i386/acpi-build.c:1138:9: warning: 'pm.cpu_hp_io_base' may be used uninitialized in this function [-Wmaybe-uninitialized]
           aml_append(crs,
           ^
  hw/i386/acpi-build.c:1666:16: note: 'pm.cpu_hp_io_base' was declared here
       AcpiPmInfo pm;
                  ^

In acpi_get_pm_info() some of the fields are pre-initialized
to 0, but this one was missed.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
