builder: Pass ~prog global (program name) around.
mllib: Add library function to run external command and slurp up the output.

This also changes a couple of functions to use this new library
function.
builder: Fix handling of --size parameter.

It now matches the documentation:

  "Select the size of the output disk, [...]

  Virt-builder will resize filesystems inside the disk image
  automatically.

  If the size is not specified, then one of two things happens.  If
  the output is a file, then the size is the same as the template.  If
  the output is a device, partition, etc then the size of that device
  is used."

This also adds checks to make sure that users don't try to shrink a
disk (which is not possible), and they don't try to resize larger than
the output block device.