vbox: Cleanup vboxAttachDrives implementation
vbox: Cleanup vboxAttachDrives implementation

This commit primes vboxAttachDrives for further changes so when they
are made, the diff is less noisy:

* move variable declarations to the top of the function
* add disk variable to replace all the def->disks[i] instances
* add cleanup at the end of the loop body, so it's all in one place
  rather than scattered through the loop body. It's purposefully
  called 'cleanup' rather than 'skip' or 'continue' because future
  commit will treat errors as hard-failures.

fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,zippy2/libvirt,zippy2/libvirt,andreabolognani/libvirt,nertpinx/libvirt,nertpinx/libvirt,nertpinx/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,eskultety/libvirt,eskultety/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,eskultety/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,eskultety/libvirt,jardasgit/libvirt,andreabolognani/libvirt