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 bd108a44 ("migration: ram: Switch to
ram block writeback").

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20191219154323.325255-1-anthony.perard@citrix.com>