memory: use atomic ops for setting dirty memory bits

Use set_bit_atomic() and bitmap_set_atomic() so that multiple threads
can dirty memory without race conditions.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1417519399-3166-4-git-send-email-stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
