block/iscsi: reenable iscsi_co_get_block_status

Commit f35c934a accidently disabled iscsi_co_get_block_status for all
libiscsi versions. Its not possible to check for enumeration constants
in the C preprocessor. This patch changes the check to the preprocessor
constant LIBISCSI_FEATURE_IOVECTOR which was introduced shortly after
get_lba_status support was added to libiscsi.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>