memory: Document that the RAM MR initializers do not handle migration

The various functions for initializing RAM MemoryRegions do not do
anything to cause the data in the MemoryRegion to be migrated.
Note in their documentation comments that this is the responsibility
of the caller.

(We will shortly add a new function that *does* do this for you.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1499438577-7674-3-git-send-email-peter.maydell@linaro.org