exec: skip MMIO regions correctly in cpu_physical_memory_write_rom_internal

Loading the BIOS in the mac99 machine is interesting, because there is a
PROM in the middle of the BIOS region (from 16K to 32K).  Before memory
region accesses were clamped, when QEMU was asked to load a BIOS from
0xfff00000 to 0xffffffff it would put even those 16K from the BIOS file
into the region.  This is weird because those 16K were not actually
visible between 0xfff04000 and 0xfff07fff.  However, it worked.

After clamping was added, this also worked.  In this case, the
cpu_physical_memory_write_rom_internal function split the write in
three parts: the first 16K were copied, the PROM area (second 16K) were
ignored, then the rest was copied.

Problems then started with commit 965eb2fc (exec: do not clamp accesses
to MMIO regions, 2015-06-17).  Clamping accesses is not done for MMIO
regions because they can overlap wildly, and MMIO registers can be
expected to perform full-width accesses based only on their address
(with no respect for adjacent registers that could decode to completely
different MemoryRegions).  However, this lack of clamping also applied
to the PROM area!  cpu_physical_memory_write_rom_internal thus failed
to copy the third range above, i.e. only copied the first 16K of the BIOS.

In effect, address_space_translate is expecting _something else_ to do
the clamping for MMIO regions if the incoming length is large.  This
"something else" is memory_access_size in the case of address_space_rw,
so use the same logic in cpu_physical_memory_write_rom_internal.

Reported-by: Alexander Graf <agraf@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Fixes: 965eb2fc
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>