pandroid: don't fork and run pinging program

This is completely unneccessary, requires a path, and causes issues on
failure if trying to load a snapshot from the command line.
sdcard prints its location if tmpfile
pandroid: if a NAND device has size 0, get size from block device API
allow forced unregistration of loadvm handlers

This is required to hook loadvm handlers; the new handler must
be the first one in the registered handlers list that matches
the device being loaded.
rr: Add callback before loadvm is called on replay

This allows us to hook arbitary devices' loadvm handlers
pandroid: Move goldfish MMC definitions to the header
vmstate: add macro for buffer with uint64 field for size
pandroid: add per-nand state to snapshot
pandroid: move sdcard and mmc device structure definitions to headers
example: add example loadvm interposition
rr: PANDA callback before DMA on replay

Currently does not cover map/unmap.
plugins: add virt-to-phys address API for plugins