acpi: fix pvpanic device is not shown in ui

Commit 2332333c added a _STA method that hides the device. The fact
that the device is not shown in the gui make it harder to install its
Windows' device.

https://bugzilla.redhat.com/show_bug.cgi?id=1238141

Signed-off-by: Gal Hammer <ghammer@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>
