acl: Fix acl_remove not to mess up the ACL

It leaks memory and fails to adjust qemu_acl member nentries.  Future
acl_add become confused: can misreport the position, and can silently
fail to add.

Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
