qemuxml2argvtest: Test localtime clock basis

qemuxml2argvtest: Test localtime clock basis

When trying to introduce a test for previous patch, I've
noticed that the command line is constructed using current
time. This won't work in our test suite (unless you guys
wants to set a specific time prior to each test run :) ).
Therefore we need to mock calls to time(2) to return the
same value every time it's called.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>