Correct backing file size comparison

The backing file size is compared with the requested size to
ensure that the requested size is larger than the backing file.
This comparison only handles integer values.  If there is a
fractional part in the backing file size, the comparison could be
performed incorrectly resulting in an image that is too small.

This patch drops off the fractional part of the backing file size
before performing the comparison.
Allow VM_POST_SCRIPT to be undefined

If one leaves VM_POST_SCRIPT undefined, setupvm.sh will exit
prematurely due to a non-zero return value from basename.  This
patch only attemps to get the basename of the post script if
VM_POST_SCRIPT is defined.
Allow cloud-init to add yum repos and packages

The VM config files currently allow one to define yum repositories
and packages to install by using the following variables:

  - VM_OS_BASE_REPO_LIST
  - VM_REPO_LIST
  - VM_PACKAGE_LIST

Unfortuantely, these only work when using a kickstart.  This patch
allows these same variables to be used to update the cloud-config
data that is used by cloud-init.  This allows yum reposities, yum
updating, and installation of additional packages to be easily
defined in the VM config file instead of resorting to handling it
in the post-cloud-init shell script.
Merge pull request #16 from nkinder/setupvm_fixes