xtf-runner: python3 fix

xtf-runner: python3 fix

issue:
  File "/home/xtf/xtf-runner", line 410, in interpret_selection
    if not line.startswith("xen_caps"):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: startswith first arg must be bytes or a tuple of bytes, not str

Adding `universal_newlines` open stdout as text file, so line should
be a `str`. `universal_newlines` is available on python 2.7. A new
alias `text` is only available in python 3.7.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>