qemu.tests.migration: Don't close remote sessions

As the pre processing code already takes care of that
anyway.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu.tests.migration: Shut down VM if killing migration bg cmd times out

Sometimes, post migration run, we can run into a condition
where tcpdump or the VM hangs at migration cleanup stage.
After analyzing test logs, including VM logs, there's nothing
that points out to any guest OS crash or similar condition.

With that in mind, it is admissible to be more lenient
when trying to kill the migration background command, and
if a timeout happens when trying to kill it, be safe and
try to shut down the vm gracefully.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu.tests.migrate: Adjust tcpdump kill command to be harsher

Use SIGKILL instead of SIGTERM (the default for pkill)
when terminating the background process. This way we're
less likely to run into a situation where killing the
tcpdump command times out.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.storage: Remove image to be backed up before copying the source

On an attempt to eliminate sources of disk corruption
and errors, delete the dst file if the src file is
avaliable.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1216 from autotest/restore-image

Make the test runner more stable with 'safe' options