This is fixed upstream by v1.2.4-71-ga4693c7:

commit a4693c76f5beb280869dac9d103ceb632c6c1403
Author: Jiri Denemark <jdenemar>
Date:   Thu May 15 13:11:12 2014 +0200

    qemu: Avoid leak in qemuDomainCheckRemoveOptionalDisk
    
    Coverity complains about event being leaked in
    qemuDomainCheckRemoveOptionalDisk. The best fix for it is to remove the
    disk directly since we already know its index.
    
    Signed-off-by: Jiri Denemark <jdenemar>

verify version:
libvirt-1.2.8-8.el7.x86_64
qemu-kvm-rhev-2.1.2-12.el7.x86_64
kernel-3.10.0-208.el7.x86_64

steps :
1.define and start a healthy domain:
# virsh list
 Id    Name                           State
----------------------------------------------------
 8     r7q2                           running

2.the domain has two disks with same source file and startupPolicy=optional
# virsh dumpxml r7q2 | grep disk -A 10
    <suspend-to-disk enabled='no'/>
  </pm>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/r7q2.img' startupPolicy='optional'/>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/r7q2.img' startupPolicy='optional'/>
      <backingStore/>
      <target dev='vdb' bus='virtio'/>
      <alias name='virtio-disk1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </disk>
3.remove the disk file
# mv r7q2.img r7q2.img.bak
# ls r7q2.img
ls: cannot access r7q2.img: No such file or directory

4.reboot the domain , domain start successfully
# virsh destroy r7q2 ; virsh start r7q2
Domain r7q2 destroyed
Domain r7q2 started
4.check the disk in domain XML , disk was dropped
# virsh dumpxml r7q2 | grep disk -A 10

domain can start successfully , it can get expected results.


Additional info:
According to libvirt.org:
For a "file" or "volume" disk type which represents a cdrom or floppy (the device attribute), it is possible to define policy what to do with the disk if the source file is not accessible. (NB, startupPolicy is not valid for "volume" disk unless the specified storage volume is of "file" type).
 
so i have a test like this:

1.prepare a pool in which the volume is of 'file' type
# virsh vol-list netfs-nfs-pool --details
 Name              Path                                                Type       Capacity  Allocation
-------------------------------------------------------------------------------------------------------
 disk.img          /var/lib/libvirt/images/netfs-nfs/disk.img          file       1.00 GiB      0.00 B
 qcow2.img         /var/lib/libvirt/images/netfs-nfs/qcow2.img         file       1.00 GiB      0.00 B
2.add the volume to domain 
# virsh dumpxml r7q2 | grep disk -A 10
<disk type='volume' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source pool='netfs-nfs-pool' volume='qcow2.img' startupPolicy='optional'/>
      <target dev='vdb' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </disk>
    <disk type='volume' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source pool='netfs-nfs-pool' volume='qcow2.img' startupPolicy='optional'/>
      <target dev='vdc' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
    </disk>

3.remove the volume 
# virsh vol-list netfs-nfs-pool --details 
 Name              Path                                                Type    Capacity  Allocation
----------------------------------------------------------------------------------------------------
 disk.img          /var/lib/libvirt/images/netfs-nfs/disk.img          file    1.00 GiB      0.00 B
 qcow2.img.bak     /var/lib/libvirt/images/netfs-nfs/qcow2.img.bak     file    1.00 GiB  196.00 KiB

4.reboot the domain , fail to start . can't find volume 
# virsh destroy r7q2 ; virsh start r7q2
Domain r7q2 destroyed

error: Failed to start domain r7q2
error: Storage volume not found: no storage vol with matching path '/var/lib/libvirt/images/qcow2.img'

Is it right use 'volume' disk type like above ? should it drop the missing volume and start successfully ?
Thanks .

Yeah, the disk should be removed even if it is specified using pool/volume. Please, file a separate bug for it.

Thanks for the info .
file a new bug 1168453 to track the issue .
and according to comment 3 , move this bug to verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0323.html