tests: Unify fake execCmd mechanism

qemuimg tests were using two mechanisms for faking utils.execCmd. The
old one was supporting faking a single command, no arguments validation,
and no way to control the retruned values of the call. This also used
textwrap module and partial functions where simpler ways can be used.

Now all tests are using the new mechanism, supporting:

- faking multiple commands, (e.g. call foo and then call bar based on
  foo return value)
- checking input arguments (e.g. was it called with -foo "bar"?)
- returning any value from the call (e.g return 0, '', '' on success,
  or return 1, '', 'error' on errors).

Sine this change requires rewriting of all the tests, I also normalized
the tests names to pep8 style, as they were very hard to read. This
could be a separate patch, but I don't think we would like to waste time
reviewing and verifying another patch for this.

Change-Id: I1b52d49047ebcced644ef39f256d8fac863a426c
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32884
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>