meson: clean up build_by_default

Build all executables by default except for the known-broken ones.

This also allows running qemu-iotests without manually building
socket_scm_helper.

Reported-by: Max Reitz <mreitz@redhat.com>
Tested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

trace/meson.build

@@ -70,7 +70,6 @@ foreach d : [
                 output: d[0],
                 input: meson.source_root() / 'trace-events',
                 command: [ tracetool, '--group=root', '--format=@0@'.format(d[1]), '@INPUT@' ],
-                build_by_default: true, # to be removed when added to a target
                 capture: true)
   specific_ss.add(gen)
 endforeach