Support --vmfile and --vmsize command line options for deploying a virtual machine.

Support --vmfile and --vmsize command line options for deploying a virtual machine.

Using the --vmfile option sets up a virtual machine instead of
deploying Debian to a block device or directory.

The --vmsize option configures the size of the virtual disk file
that's being deployed.

Thanks to Thorsten Glaser for help with installing Grub accordingly.