exec: Implement subpage_read/write via address_space_rw

This will allow to add support for unaligned memory regions: the subpage
container region can activate unaligned support unconditionally because
the read/write handler will now ensure that accesses are split as
required by calling address_space_rw. We can furthermore drop the
special handling of RAM subpages, address_space_rw takes care of this
already.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>