pc: acpi: fix pvpanic for buggy guests

pc: acpi: fix pvpanic for buggy guests

In the old times, we always had pvpanic in ACPI and a _STA method told
the guest not to use it.  Automatic generation dropped the _STA method
as the specification says that missing _STA means enabled and working.
Some guests (Linux) had buggy drivers and this change made them unable
to utilize pvpanic.

A Linux patch is posted as well, but I think it's worth to make pvpanic
useable on old guests at the price of three lines and few bytes of SSDT.

The old _STA method was
  Method (_STA, 0, NotSerialized) {
      Store (PEST, Local0)
      If (LEqual (Local0, Zero)) {
          Return (Zero) }
      Else {
          Return (0x0F) }}

Igor pointed out that we don't need to use a method to return a constant
and that 0xB (don't show in UI) is the common definition now.

Also, the device used to be PEVT.  (PEVT as in "panic event"?)

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>