qemu_passt: Deduplicate passt killing code

qemu_passt: Deduplicate passt killing code

There are two places where we kill passt:

1) qemuPasstStop() - called transitively from qemuProcessStop(),
2) qemuPasstStart() - after failed start.

Now, the code from 2) lack error preservation (so if there's
another error during cleanup we might overwrite the original
error). Therefore, move the internals of qemuPasstStop() into a
separate function and call it from both places.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>