exec: fix incorrect assumptions in memory_access_size

access_size_min can be 1 because erroneous accesses must not crash
QEMU, they should trigger exceptions in the guest or just return
garbage (depending on the CPU).  I am not sure I understand the
comment: placing a 4-byte field at the last byte of a region
makes no sense (unless impl.unaligned is true), and that is
why memory.c:access_with_adjusted_size does not bother with
minimums larger than the remaining length.

access_size_max can be mr->ops->valid.max_access_size because memory.c
can and will still break accesses bigger than
mr->ops->impl.max_access_size.

Reported-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
