conf: domain: Parse backingStore with VIR_DOMAIN_DEF_PARSE_DISK_SOURCE

conf: domain: Parse backingStore with VIR_DOMAIN_DEF_PARSE_DISK_SOURCE

The only code path which calls the parser with the
VIR_DOMAIN_DEF_PARSE_DISK_SOURCE is from qemuDomainBlockCopy. Since that
code path can properly handle backing chains for the disk and it's
desired to pass the parsed chains to the block copy code remove the
condition which prevents parsing the <backingStore> element.

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