conf: Introduce pstore device

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>