qemu_vm: Log the original traceback when VM.create() failes

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.

This patch requires updated version of autotest!

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