arch_init: right return for ram_save_iterate

qemu_file_rate_limit() never return negative value since the refactor
by Commit 1964a39, this patch gets rid of the negative check for it,
adjust bytes_transferred and return value correspondingly in
ram_save_iterate().

Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
