zdtm: Call criu.available() only for `run` action

When zdtm.py is executed with `list` sub-command the 'criu_bin'
option is not defined and criu.available() fails.

$ python test/zdtm.py list
Traceback (most recent call last):
  File "test/zdtm.py", line 2243, in <module>
    criu.available()
  File "test/zdtm.py", line 1185, in available
    if not os.access(opts['criu_bin'], os.X_OK):
KeyError: u'criu_bin'

However, we don't need to check the existence of criu_bin
unless we use the `run` action.

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>
Acked-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>