run_utah_tests.py silently fails when virtualbox is running

[Javier Collado]

KVM and virtualbox shouldn't be executed together. When this happens, run_utah_tests.py fails and no error message is printed.

Debugging the code, it can be seen that an exception is raised inside an except clause with the following message:
libvirtError('internal error process exited while connecting to monitor: failed to initialize KVM: Device or resource busy\nNo accelerator found!\n',)

A check should be added to warn users that they shouldn't try to run both KVM and virtualbox at the same time (possibly vmware too).