tests/migration: introduce multifd into guestperf

tests/migration: introduce multifd into guestperf

Guestperf tool does not cover the multifd-enabled migration
currently, it is worth supporting so that developers can
analysis the migration performance with all kinds of
migration.

To request that multifd is enabled, with 4 channels:
$ ./tests/migration/guestperf.py \
    --multifd --multifd-channels 4 --output output.json

To run the entire standardized set of multifd-enabled
comparisons, with unix migration:
$ ./tests/migration/guestperf-batch.py \
    --dst-host localhost --transport unix \
    --filter compr-multifd* --output outputdir

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
Message-Id: <cfeeb04d17ad932c42a9871294058b77429ad1b7.1616171924.git.huangy81@chinatelecom.cn>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>