pvpanic : update pvpanic spec document

Add pvpanic PCI device support details in docs/specs/pvpanic.txt.

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/specs/pvpanic.txt

 PVPANIC DEVICE
 ==============
 
-pvpanic device is a simulated ISA device, through which a guest panic
+pvpanic device is a simulated device, through which a guest panic
 event is sent to qemu, and a QMP event is generated. This allows
 management apps (e.g. libvirt) to be notified and respond to the event.
 
@@ -9,6 +9,9 @@ The management app has the option of waiting for GUEST_PANICKED events,
 and/or polling for guest-panicked RunState, to learn when the pvpanic
 device has fired a panic event.
 
+The pvpanic device can be implemented as an ISA device (using IOPORT) or as a
+PCI device.
+
 ISA Interface
 -------------
 
@@ -24,6 +27,14 @@ bit 1: a guest panic has happened and will be handled by the guest;
        the host should record it or report it, but should not affect
        the execution of the guest.
 
+PCI Interface
+-------------
+
+The PCI interface is similar to the ISA interface except that it uses an MMIO
+address space provided by its BAR0, 1 byte long. Any machine with a PCI bus
+can enable a pvpanic device by adding '-device pvpanic-pci' to the command
+line.
+
 ACPI Interface
 --------------