KVM: ia64: fix dirty_log_lock spin_lock section not to include get_dirty_log()

kvm_get_dirty_log() calls copy_to_user(). So we need to narrow the
dirty_log_lock spin_lock section not to include this.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>