avocado.virt.qemu.machine: Start QEMU process after init

Due to the changes in the avocado.process.SubProcess() class,
now the SubProcess object has to be started after it is
initialized. Let's reflect that in the Machine() class.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #13 from lmr/fix-qemu-startup

avocado.virt.qemu.machine: Start QEMU process after init