hw/dma: Move ScatterGatherEntry / QEMUSGList declarations around

hw/dma: Move ScatterGatherEntry / QEMUSGList declarations around

In the next commit we will use the dma_addr_t type in the QEMUSGList
structure. Since currently dma_addr_t is defined after QEMUSGList,
move the declarations to have dma_addr_t defined first. This is a
pure code-movement patch.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220111184309.28637-10-f4bug@amsat.org>