conf: Introduce pstore device

The aim of pstore device is to provide a bit of NVRAM storage for
guest kernel to record oops/panic logs just before the it
crashes. Typical usage includes usage in combination with a
watchdog so that the logs can be inspected after the watchdog
rebooted the machine. While Linux kernel (and possibly Windows
too) support many backends, in QEMU there's just 'acpi-erst'
device so stick with that for now. The device must be attached to
a PCI bus and needs two additional values (well, corresponding
memory-backend-file needs them): size and path. Despite using
memory-backend-file this does NOT add any additional RAM to the
guest and thus I've decided to expose it as another device type
instead of memory model.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>

docs/formatdomain.rst

@@ -8659,6 +8659,38 @@ The optional attribute ``backend`` is required if the ``type`` is ``qemu``, the
    ...
 
 
+Pstore
+~~~~~~~~~
+
+Pstore is an oops/panic logger that writes its logs to a block device and
+non-block device before the system crashes. Currently only ACPI Error Record
+Serialization Table, ERST, is supported. This feature is designed for storing
+error records in persistent storage for future reference and/or debugging.
+:since:`Since v10.6.0`
+
+::
+
+  ...
+  <pstore backend='acpi-erst'>
+    <path>/tmp/guest_acpi_esrt</path>
+    <size unit='KiB'>8</size>
+    <address type='pci' domain='0x0000' bus='0x02' slot='0x01' function='0x0'/>
+  </pstore>
+  ...
+
+The ``pstore`` element has one mandatory attribute ``backend`` which selects
+desired backend (only ``acpi-erst`` is accepted for now). Then it has the
+following child elements:
+
+``path``
+  Represents a path in the host that backs the pstore device in the guest. It
+  is mandatory.
+
+``size``
+  Configures the size of the persistent storage available to the guest. It is
+  mandatory.
+
+
 Security label
 --------------