Description of problem:
After revert to a shutdown snapshot, domain start will hang for 7-13 minute on my test. The domain I use is rhel6/6.1.

Version-Release number of selected component (if applicable):
kernel-2.6.32-125.el6.x86_64
qemu-kvm-0.12.1.2-2.153.el6.x86_64
libvirt-0.8.7-15.el6.x86_64


How reproducible:
100%

Steps to Reproduce:
1. Prepare a shutdown domain with qcow2 img
2. create a snapshot when domain is shutdown
virsh snapshot-create domain 
3. start the domain
virsh start domain
4. revert domain to a shutdown snapshot
virsh snapshot-revert domain snapshot-name
5. start domain
virsh start domain
  
Actual results:
virsh start will hang for several minutes

Expected results:
virsh start will succeed with no hang

Additional info:
domain xml
<domain type='kvm' id='21'>
  <name>vr-rhel6u1-x86_64-kvm</name>
  <uuid>468d7413-1e95-a3c2-caba-834af5736659</uuid>
  <memory>524288</memory>
  <currentMemory>524288</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.1.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/vr-rhel6u1-x86_64-kvm'/>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>
    <controller type='ide' index='0'>
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <interface type='network'>
      <mac address='52:54:00:37:50:c2'/>
      <source network='default'/>
      <target dev='vnet2'/>
      <model type='rtl8139'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <source path='/dev/pts/5'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/5'>
      <source path='/dev/pts/5'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='5902' autoport='yes' keymap='en-us'/>
    <sound model='ac97'>
      <alias name='sound0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </sound>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </memballoon>
  </devices>
  <seclabel type='dynamic' model='selinux'>
    <label>system_u:system_r:svirt_t:s0:c473,c587</label>
    <imagelabel>system_u:object_r:svirt_image_t:s0:c473,c587</imagelabel>
  </seclabel>
</domain>