Description of problem:
Hot-plug of VNIC fails, on new VM i can hot-plug new VNIC but after I remove that VNIC and try to add another VNIC using hot-plug it fails.


Version-Release number of selected component (if applicable):
VDSM - vdsm-4.10.3-0.416.git5358ed2.el6.x86_64
libvirt - libvirt-0.10.2-18.el6_4.5.x86_64


Steps to Reproduce:
1. Create VM and run it.
2. hot-plug VNIC to the VM
3. un hot-plug the VNIC from the VM
4. Add another VNIC to the VM using hot-plug

Actual results:
Add VNIC fails

Expected results:
Add VNIC should succeed

Additional info:
Thread-1552::DEBUG::2013-05-27 13:34:04,961::libvirtvm::1559::vm.Vm::(hotplugNic) vmId=`17871f50-42e6-453e-ae43-8962125b5fb2`::Hotplug NIC xml: <interface type="bridge">
        <mac address="00:1a:4a:16:88:57"/>
        <model type="virtio"/>
        <source bridge="rhevm"/>
        <filterref filter="vdsm-no-mac-spoofing"/>
        <link state="up"/>
</interface>

Dummy-95::DEBUG::2013-05-27 13:34:05,185::storage_mailbox::727::Storage.Misc.excCmd::(_checkForMail) 'dd if=/rhev/data-center/644e5f7c-9cf3-4b77-877d-37fd655e8d0b/mastersd/dom_md/inbox iflag=
direct,fullblock count=1 bs=1024000' (cwd None)
Dummy-95::DEBUG::2013-05-27 13:34:05,327::storage_mailbox::727::Storage.Misc.excCmd::(_checkForMail) SUCCESS: <err> = '1+0 records in\n1+0 records out\n1024000 bytes (1.0 MB) copied, 0.049298
1 s, 20.8 MB/s\n'; <rc> = 0
Thread-1552::DEBUG::2013-05-27 13:34:05,362::libvirtconnection::136::vds::(wrapper) Unknown libvirterror: ecode: 1 edom: 0 level: 2 message: An error occurred, but the cause is unknown
Thread-1552::ERROR::2013-05-27 13:34:05,363::libvirtvm::1564::vm.Vm::(hotplugNic) vmId=`17871f50-42e6-453e-ae43-8962125b5fb2`::Hotplug failed
Traceback (most recent call last):
  File "/usr/share/vdsm/libvirtvm.py", line 1562, in hotplugNic
    self._dom.attachDevice(nicXml)
  File "/usr/share/vdsm/libvirtvm.py", line 546, in f
    ret = attr(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/vdsm/libvirtconnection.py", line 112, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 399, in attachDevice
    if ret == -1: raise libvirtError ('virDomainAttachDevice() failed', dom=self)
libvirtError: An error occurred, but the cause is unknown