support disk snapshots with emulator

Being able to create a snapshot and roll back to it is very useful for developers. Eg, developers can
 * develop features, etc on the image, then package them up, roll back the image and test on a pristine image
 * save time on system-image upgrade testing
 * do multiple test runs, reverting to the pristine image between runs to cut down on variances
 * verify bugs in the image easily
 * etc

Snapshotting can be complicated, but most of the time you just want to have a pristine snapshot that you can rollback to with the option of committing changes. Attached is an updated run-emulator.sh script to achieve this:
$ ./run-emulator.sh help

Usage: run-emulator.sh
       run-emulator.sh [COMMAND]

Commands:
 use-disk-snapshots Convert images to qcow2 and create 'pristine' snapshot
 convert-disk DISK Convert DISK to qcow2 and create 'pristine' snapshot
 snapshot-disks Update 'pristine' snapshot to current state
 revert-disks Revert current images to 'pristine' snapshot
 info-disks Show disk information for current images

Without arguments, run the emulated image.

Note: these commands only manipulate the qemu qcow2 disk images and are
      unrelated to the -snapshot* and -no-snapshot* emulator command line
      options

Basic workflow is:
$ ./build-emulator-sdcard.sh # create image like normal, skip if already have image
$ ./run-emulator.sh use-disk-snapshots
$ ./run-emulator.sh
<do work>
$ ./run-emulator.sh revert-disks
$ ./run-emulator.sh
<perform tests in pristine environment>
$ ./run-emulator.sh revert-disks

Once system-image is supported, can update the image like so:
$ ./run-emulator.sh revert-disks
$ ./run-emulator.sh
<run system-image>
$ ./run-emulator.sh snapshot-disks

The emulator is based on qemu 0.12 and the default disk compatibility option for qcow2 images is 0.10 (though I explicitly use compat=0.10 in the script). The script continues to run as before with raw images and snapshotting is only usable after running 'use-disk-snapshots'. When using this command to run-emulator.sh, disks specified with '-system', 'data', '-cache' and '-sdcard' are all converted to qcow2 and updated to have a 'pristine' snapshot. 'convert-disk DISK' can be used to convert a single disk, which might be useful when rerunning build-emulator-sdcard.sh.

Note that this script only deals with the disk files themselves and adds a dependency on qemu-utils. It does not require advanced functionality with the emulator and is lightly tested. The emulator itself has other snapshot commands that might be interesting to expose to users (eg, /usr/share/android/emulator/out/host/linux-x86/bin/emulator -help|grep snap). This page could provide inspiration for further enhancements: http://stackoverflow.com/questions/4842612/how-do-you-save-android-emulator-snapshot/5980578#5980578. I believe the above simple disk commands are complementary to the emulator snapshot commands and possibly more desirable for the simple use cases I outlined above.

Feel free to adjust as desired.