avocado.virt.qemu.machine: Fix serial console not being recorded

We should remove the serial console file only when the VM
instance is closed.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #20 from lmr/fix-serial-console

avocado.virt.qemu.machine: Fix serial console not being recorded