build: avoid 'index' as variable name

build: avoid 'index' as variable name

Once again, gcc 4.4.7 (hello RHEL) rears its ugly head:

conf/domain_conf.c: In function 'virDomainDiskBackingStoreFormat':
conf/domain_conf.c:14940: error: declaration of 'index' shadows a global declaration [-Wshadow]
/usr/include/string.h:489: error: shadowed declaration is here [-Wshadow]

* src/conf/domain_conf.c (virDomainDiskBackingStoreFormat): Pacify
older gcc.

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