qemu.tests: Add migration_with_netperf.py test script.

During migration, run Netperf Server inside guest and
run N* netperf clients in host.
By default, run 100 netperf clients.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.cfg: Add migrate.with_netperf sub test case.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests: Bug fix and update in migration_with_netperf.py

This patch re-format this test script.
Use utils_misc.wait_for instead of hard code sleep 20.
Add cleanup code.
Use error.context.
Remove unused import

Signed-off-by: Feng Yang <fyang@redhat.com>