qemu: Allow hotplug of vhost-scsi device

qemu: Allow hotplug of vhost-scsi device

Adjust the device string that is built for vhost-scsi devices so that it
can be invoked from hotplug.

From the QEMU command line, the file descriptors are expect to be numeric only.
However, for hotplug, the file descriptors are expected to begin with at least
one alphabetic character else this error occurs:

  # virsh attach-device guest_0001 ~/vhost.xml
  error: Failed to attach device from /root/vhost.xml
  error: internal error: unable to execute QEMU command 'getfd':
  Parameter 'fdname' expects a name not starting with a digit

We also close the file descriptor in this case, so that shutting down the
guest cleans up the host cgroup entries and allows future guests to use
vhost-scsi devices.  (Otherwise the guest will silently end.)

Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>