Handle VIR_STORAGE_TYPE_DIR in qemuBlockStorageSourceGetBackendProps so
that a 'vvfat' driver is used, which emulates a FAT filesystem
containing the folders.

qemu requires us to add it as a storage layer, since a 'raw' layer is
usually put on top of it.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>