Corrected to support Chef 11

Need to Array.new vs simply casting to Array.  Also, tests would
not pass with the recent grub/lxc work.  Was hoping to stub
`#set_boot_kernel_and_trigger_reboot` but found that to be
difficult.  Ended up stubbing the result of File.open.  This
section needs better testing.
  Failure/Error: @chef_run.converge "openstack-compute::libvirt"
  Errno::ENOENT:
   No such file or directory - /boot/grub/menu.lst
  # ./.cookbooks/openstack-compute/recipes/libvirt.rb:34:in `initialize'
  # ./.cookbooks/openstack-compute/recipes/libvirt.rb:34:in `open'
  # ./.cookbooks/openstack-compute/recipes/libvirt.rb:34:in `set_boot_kernel_and_trigger_reboot'
  # ./.cookbooks/openstack-compute/recipes/libvirt.rb:79:in `from_file'
  # ./.cookbooks/openstack-compute/spec/libvirt-opensuse_spec.rb:10:in `block (3 levels) in <top (required)>'

Change-Id: Ie97309ff5e1bf30a02714597fe29c1d0df1b1c23
Merge "Corrected to support Chef 11"