iotests/026: Move v3-exclusive test to new file

iotests/026: Move v3-exclusive test to new file

data_file does not work with v2, and we probably want 026 to keep
working for v2 images.  Thus, open a new file for v3-exclusive error
path test cases.

Fixes: 81311255f217859413c94f2cd9cebf2684bbda94
       (“iotests/026: Test EIO on allocation in a data-file”)
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200311140707.1243218-1-mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Tested-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit c264e5d2f9f5d73977eac8e5d084f727b3d07ea9)
 Conflicts:
	tests/qemu-iotests/group
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>