Whenever the MRU cache hits for the list of RAM blocks, qemu_get_ram_block
does an unnecessary write that causes a processor cache line to bounce
from one core to another.  This causes a performance hit.
Reported-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>