Description of problem:
VDSM keeps reporting (vdsm.log) that the QEMU driver does not support <metadata> element:

Thread-143::ERROR::2014-07-01 10:58:33,631::vm::319::vm.Vm::(_sampleCpuTune) vmId=`d4eaecc5-ca2f-4a04-b633-3be7c57ed2c2`::Failed to retrieve QoS metadata
Traceback (most recent call last):
  File "/usr/share/vdsm/virt/vm.py", line 316, in _sampleCpuTune
    libvirt.VIR_DOMAIN_METADATA_ELEMENT, METADATA_VM_TUNE_URI, 0)
  File "/usr/share/vdsm/virt/vm.py", line 604, in f
    ret = attr(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/vdsm/libvirtconnection.py", line 111, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 904, in metadata
    if ret is None: raise libvirtError ('virDomainGetMetadata() failed', dom=self)
libvirtError: argument unsupported: QEMU driver does not support <metadata> element

...

Thread-143::ERROR::2014-07-01 10:58:48,652::vm::319::vm.Vm::(_sampleCpuTune) vmId=`d4eaecc5-ca2f-4a04-b633-3be7c57ed2c2`::Failed to retrieve QoS metadata
Traceback (most recent call last):
  File "/usr/share/vdsm/virt/vm.py", line 316, in _sampleCpuTune
    libvirt.VIR_DOMAIN_METADATA_ELEMENT, METADATA_VM_TUNE_URI, 0)
  File "/usr/share/vdsm/virt/vm.py", line 604, in f
    ret = attr(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/vdsm/libvirtconnection.py", line 111, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 904, in metadata
    if ret is None: raise libvirtError ('virDomainGetMetadata() failed', dom=self)
libvirtError: argument unsupported: QEMU driver does not support <metadata> element
VM Channels Listener::DEBUG::2014-07-01 10:58:48,794::vmchannels::96::vds::(_handle_timeouts) Timeout on fileno 49.


Version-Release number of selected component (if applicable):
vdsm-4.16.0-7.git15d7b11.el6.x86_64
libvirt-0.10.2-29.el6_5.9.x86_64
qemu-kvm-rhev-0.12.1.2-2.415.el6_5.3.x86_64


How reproducible:
100%

Steps to Reproduce:
1. Start a VM

Actual results:
The error message "QEMU driver does not support <metadata> element" is logged every 15 seconds.

Expected results:
If this is a soft dependency and it's expected that on some platforms the QEMU driver is not supporting <metadata> element, then we should decrease the log verbosity (no traceback, debug/info level, maybe report only once).