hw/misc/pvpanic: split-out generic and bus dependent code

hw/misc/pvpanic: split-out generic and bus dependent code

To ease the PCI device addition in next patches, split the code as follows:
- generic code (read/write/setup) is being kept in pvpanic.c
- ISA dependent code moved to pvpanic-isa.c

Also, rename:
- ISA_PVPANIC_DEVICE -> PVPANIC_ISA_DEVICE.
- TYPE_PVPANIC -> TYPE_PVPANIC_ISA.
- MemoryRegion io -> mr.
- pvpanic_ioport_* in pvpanic_*.

Update the build system with the new files and config structure.

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>