virttest.qemu_vm: Fix issue #381

It was correctly pointed out by ldoktor that, in case
the 9p proxy helper is set, we'll overwrite self.process
with the regular QEMU process. Let's fix this problem.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1179 from autotest/fix-381

virttest.qemu_vm: Fix issue #381