qemu.tests.virtio_console: Fix incorrect params problem and negative test issues

Hi guys,

this pull request fixes some issues of (not only) virtio_console test.

I tried to solve the ad132b0 `Force new params in vm.create()` one in framework, but I haven't found any way to fix this. There might be other places, where this problem exists and there is no way to identify them easily. The trouble ones are the ones which use `start_vm = no` and call `vm.create()` directly without calling `env.preprocess()` or with `env.preprocess()` called while `start_vm = no`. (the ones which modifies params to `start_vm = yes` and then calls `env.preprocess()` works fine)

The last patch returns fixes the issue where `vm.create()` use to return the qemu output, but the commit cdeed8a3 added one branch which returned custom message instead. This caused some negative tests failures.

Kind regards,
Lukáš