virttest.utils_misc: Add log_last_traceback function

This is a copy of function from autotest.client.shared.base_utils.
Keep it here for few weeks in order to allow users with old autotest
run the current virttest.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_vm: Log the original traceback when VM.create() fails

When self.make_create_command() raises an exception in VM.create()
it's catched and reraised. The problem is that the information about
actual failed line is lost. This patch logs the original traceback
and raise VMStartError instead.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>