misc/pvpanic: deduplicate common code

pvpanic-mmio.c and pvpanic-pci.c share a lot of code.
Refactor it into pvpanic.c where it doesn't have to be kept in sync
manually and where the core logic can be understood more easily.

No functional change.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20231011-pvpanic-cleanup-v2-1-4b21d56f779f@weissschuh.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>