drive_get_index() returns -1 if a drive isn't found; don't
use -1 to index drives_table.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5719 c046a42c-6fe2-441c-8c8c-71466251a162