acl: acl_add can't insert before last list element, fix

Watch this:

    $ upstream-qemu -nodefaults -S -vnc :0,acl,sasl -monitor stdio
    QEMU 1.5.50 monitor - type 'help' for more information
    (qemu) acl_add vnc.username drei allow
    acl: added rule at position 1
    (qemu) acl_show vnc.username
    policy: deny
    1: allow drei
    (qemu) acl_add vnc.username zwei allow 1
    acl: added rule at position 2
    (qemu) acl_show vnc.username
    policy: deny
    1: allow drei
    2: allow zwei
    (qemu) acl_add vnc.username eins allow 1
    acl: added rule at position 1
    (qemu) acl_show vnc.username
    policy: deny
    1: allow eins
    2: allow drei
    3: allow zwei

The second acl_add inserts at position 2 instead of 1.

Root cause is an off-by-one in qemu_acl_insert(): when index ==
acl->nentries, it appends instead of inserting before the last list
element.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
