Memory: Only call ramblock_ptr when needed in qemu_ram_writeback

Memory: Only call ramblock_ptr when needed in qemu_ram_writeback

It is possible that a ramblock doesn't have memory that QEMU can
access, this is the case with the Xen hypervisor.

In order to avoid to trigger an assert, only call ramblock_ptr() when
needed in qemu_ram_writeback(). This should fix migration of Xen
guests that was broken with bd108a44bc29 ("migration: ram: Switch to
ram block writeback").

Signed-off-by: Anthony PERARD <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Message-Id: <20191219154323[email protected]>