conf: storage: also sanitize source dir

conf: storage: also sanitize source dir

Commit a7fb2258 added sanitization of storage pool target paths,
however source dir paths were left unsanitized.

A netfs pool with:
<source>
  <host name='10.20.30.40'/>
  <dir path='/nfs/'/>
</source>
will not be correctly detected as mounted by
virStorageBackendFileSystemIsMounted, because it shows up in the
mount list without the trailing slash.

Sanitize the source dir as well.

https://bugzilla.redhat.com/show_bug.cgi?id=1723247

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>