exec: hide mr->ram_addr from qemu_get_ram_ptr users

Let users of qemu_get_ram_ptr and qemu_ram_ptr_length pass in an
address that is relative to the MemoryRegion.  This basically means
what address_space_translate returns.

Because the semantics of the second parameter change, rename the
function to qemu_map_ram_ptr.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>