qemu: Remove extra messages from virtio-scsi hotplug
    
    I tried to attach a SCSI LUN to two different guests, and forgot
    to specify "shareable" in the hostdev XML.  Attaching the device
    to the second guest failed, but the message was not helpful in
    telling me what I was doing wrong:
    
      $ cat scsi_scratch_disk.xml
        <hostdev mode='subsystem' type='scsi'>
          <source>
            <adapter name='scsi_host3'/>
            <address bus='0' target='15' unit='1074151456'/>
          </source>
        </hostdev>
    
      $ virsh attach-device dasd_sles_d99c scsi_scratch_disk.xml
      Device attached successfully
    
      $ virsh attach-device dasd_fedora_0e1e scsi_scratch_disk.xml
      error: Failed to attach device from scsi_scratch_disk.xml
      error: internal error: Unable to prepare scsi hostdev: scsi_host3:0:15:1074151456
    
    I eventually discovered my error, but thought it was weird that
    Libvirt doesn't provide something more helpful in this case.
    Looking over the code we had just gone through, I commented out
    the "internal error" message, and got something more useful:
    
      $ virsh attach-device dasd_fedora_0e1e scsi_scratch_disk.xml
      error: Failed to attach device from scsi_scratch_disk.xml
      error: Requested operation is not valid: SCSI device 3:0:15:1074151456 is already in use by other domain(s) as 'non-shareable'
    
    Looking over the error paths here, we seem to issue better
    messages deeper in the callchain so these "internal error"
    messages overwrite any of them.  Remove them, so that the
    more detailed errors are seen.
    
    Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>