virt.storage: Fix bug when checking for backup directory

By default, .get() on a dict will return None. Using None
in a os.path related function is going to throw an error.

So we can fix that issue by using an appropriate default
value that won't break os.path.isabs().

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1284 from autotest/fix-bug-storage

virt.storage: Fix bug when checking for backup directory