run_migration: Implement our own wait

Bash 5.0 changed 'wait' with no arguments to also wait for all
process substitutions. For example, with Bash 4.4 this completes,
after waiting for the sleep

  (
    sleep 1 &
    wait
  ) > >(tee /dev/null)

but with Bash 5.0 it does not. The kvm-unit-tests (overly) complex
bash scripts have a 'run_migration ... 2> >(tee /dev/stderr)'
where the '2> >(tee /dev/stderr)' comes from 'run_qemu'. Since
'run_migration' calls 'wait' it will never complete with Bash 5.0.
Resolve by implementing our own wait; just a loop on job count.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20200404154739.217584-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>