iotests: validate dmsetup result in test 128

The I/O test 128 uses 'dmsetup create' to create a device, optionally
using sudo to elevate privileges.

This dmsetup command works in GitLab CI, however, the test then fails
with a missing device name:

  1..1
  # running raw 128
  not ok raw 128
  ----------------------------------- stderr -----------------------------------
  --- /builds/berrange/qemu/tests/qemu-iotests/128.out
  +++ /builds/berrange/qemu/build/scratch/raw-file-128/128.out.bad
  @@ -1,5 +1,5 @@
   QA output created by 128

   == reading from error device ==
  -read failed: Input/output error
  +qemu-io: can't open device /dev/mapper/eiodev16546: Could not open '/dev/mapper/eiodev16546': No such file or directory
   *** done

  (test program exited with status code 1)

It is believed that this is due to the build env using a manually
populated /dev, such that the device mapper node won't ever appear.
It is not a race, since a test adding a sleep did not result in the
device appearing.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemu-iotests/128

@@ -42,6 +42,12 @@ _setup_eiodev()
 		echo "0 $((1024 * 1024 * 1024 / 512)) error" | \
 			$cmd dmsetup create "$devname" 2>/dev/null
 		if [ "$?" -eq 0 ]; then
+			DEV="/dev/mapper/$devname"
+			if ! -e $DEV
+			then
+				_notrun "Device $DEV not appearing"
+			fi
+
 			sudo="$cmd"
 			return
 		fi