Only building external/qemu for goldfish

This is temporary, we really need to get external/qemu outside
of the default manifest, this can be reverted later.

Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>
Merge commit 'bddd1762008de8067cbe9d0bbdf93fa0116c1e75' into phablet-10.1