exec: do not use error_mem_read

We will soon reach this case when doing (unaligned) accesses that
span partly past the end of memory.  We do not want to crash in
that case.

unassigned_mem_ops and rom_mem_ops are now the same.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
