test: Kill VM when _qemu_start() raises an exception
test: Kill VM when _qemu_start() raises an exception

Otherwise we have stragler VMs hanging around, and if these
are specific flavors, then this affects following tests.

Closes #3048
Reviewed-by: Marius Vollmer <bb11c6e942c5cc765df5f04da146b3198127a7a3@redhat.com>

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