scripts: Separate Capstan and VM image building

This separates Capstan and VM image building into two scripts. This is
needed to be able to drop useless VM images like "osv-base" or
"osv-openjdk" that only make sense as Capstan base images, not
standalone VM images that people boot to.

Cc: Tzach Livyatan <tzach@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
mmu: fix madvice(MADV_DONTNEED)

Fix madvice(MADV_DONTNEED) to use correct page ops for a VMA. Don't try
to use what we have today on mmaped files.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>