runtime: Always honor the unittests.cfg accel requirement

If the unittests.cfg file specifies an accel parameter then don't
let the user override it.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20200404154739.217584-3-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>