Move the PhysicalDiskFormat type from KVMPhysicalDisk to QemuImg

This way we have one point where we keep track of the Qemu disk formats
instead of multiple points in the code.
Add more PhysicalDiskFormat types
Remove main method from Script util.

No need for this piece of code, probably legacy from somewhere
Remove redundant runSimpleBashScript code.

It can call itself with a timeout set to 0 since
the constructor will set that to the default timeout.
Implement QemuImg.create()