target-arm: Use Neon for zero checking

Use Neon instructions to perform zero checking of
buffer. This is helps in reducing total migration time.

Use case: Idle VM live migration with 4 VCPUS and 8GB ram
running CentOS 7.

Without Neon, the Total migration time is 3.5 Sec

Migration status: completed
total time: 3560 milliseconds
downtime: 33 milliseconds
setup: 5 milliseconds
transferred ram: 297907 kbytes
throughput: 685.76 mbps
remaining ram: 0 kbytes
total ram: 8519872 kbytes
duplicate: 2062760 pages
skipped: 0 pages
normal: 69808 pages
normal bytes: 279232 kbytes
dirty sync count: 3

With Neon, the total migration time is 2.9 Sec

Migration status: completed
total time: 2960 milliseconds
downtime: 65 milliseconds
setup: 4 milliseconds
transferred ram: 299869 kbytes
throughput: 830.19 mbps
remaining ram: 0 kbytes
total ram: 8519872 kbytes
duplicate: 2064313 pages
skipped: 0 pages
normal: 70294 pages
normal bytes: 281176 kbytes
dirty sync count: 3

Signed-off-by: Vijaya Kumar K <vijayak@cavium.com>
Signed-off-by: Suresh <ksuresh@cavium.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1467190029-694-2-git-send-email-vijayak@cavium.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
