rust: restrict missing_const_for_fn to qemu_api crate

rust: restrict missing_const_for_fn to qemu_api crate

missing_const_for_fn is not necessarily useful or good.  For example in
a private API you can always add const later, and in a public API
it can be unnecessarily restrictive to annotate everything with const
(blocking further improvements to the API).

Nevertheless, QEMU turns it on because qemu_api uses const quite
aggressively and therefore it can be handy to have as much as possible
annotated with const.  Outside qemu_api though, not so much: devices
are self contained consumers and if there is nothing that could use
their functions in const contexts that were not anticipated.

Since missing_const_for_fn can be a bit noisy and trigger on trivial
functions that no one would ever call in const context, do not
turn it on everywhere and only keep it in qemu_api as a special case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>