conf: fix backing store parse off-by-one

conf: fix backing store parse off-by-one

Commit 546154e parses the type attribute from a <backingStore>
element, but forgot that the earlier commit 9673418 added a
placeholder element in the same 1.2.3 release; as a result,
the C code was mistakenly allowing "none" as a type.

Similarly, the same commit allows "none" as the <format>
sub-element type, even though that has been a placeholder
since the 0.10.2 release with commit f772b3d.

* src/conf/domain_conf.c (virDomainDiskBackingStoreParse): Require
non-zero types.

Signed-off-by: Eric Blake <eblake@redhat.com>