meson: convert the speed tests

Use meson benchmark() for them, adjust mtest2make.py for that.
A new target "make bench" can be used to run all benchmarks.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200828110734.1638685-14-marcandre.lureau@redhat.com>
[Rewrite mtest2make part. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>