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>