fix resize of unpartitioned images with libguestfs

Following on from I9c974e138ff90e8b7a5a40f5b31dcdb25a59622d
Ensure that the libguestfs path also throws a NovaException,
handled by can_resize_fs().

* nova/virt/disk/vfs/guestfs.py (setup): Move the debug message
and guestfs handle outside the exception handler as we
don't want to map failure of those to a NovaException
that indicates an issue with the image as opposed to the
nova code or libguestfs installation.  Change to a more explicit
exception thrown by guestfs, so as to avoid masking other issues.
(teardown): Remove redundant calls to str().
* nova/virt/disk/api.py (can_resize_fs): Cleanup the debug messages,
and use the newer vfs API directly, rather than the slightly
hacky call to inject_data() with no data to inject.

Fixes bug: 1094373
Change-Id: I3e1305cf6bb64278a8caf37e4c5005cb9683f632
Merge "fix resize of unpartitioned images with libguestfs"