guestfish -i and virt-inspector work on filenames containing spaces (RHBZ#507810).

This commit fixes a long-standing bug which prevented guestfish -i
and virt-inspector from working on disk images which had a space
in the filename (or other unsafe characters).  It works by ensuring
that the strings passed between guestfish -i and virt-inspector are
quoted correctly in both directions.

Note that this commit adds a dependency from virt-inspector to
the perl module String::ShellQuote.  We have previously used this
module in virt-make-fs.