qdev: make properties array "const"

qdev: make properties array "const"

Constify all accesses to qdev properties, except for the
ObjectPropertyAccessor itself.  This makes it possible to place them in
read-only memory, and also lets Rust bindings switch from "static mut"
arrays to "static"; which is advantageous, because mutable statics are
highly discouraged.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>