kvm: remove special handling of DIRTY_MEMORY_MIGRATION in the dirty log mask

One recent example is commit 4cc856fa (kvm-all: Sync dirty-bitmap from
kvm before kvm destroy the corresponding dirty_bitmap, 2015-04-02).
Another performance problem is that KVM keeps tracking dirty pages
after a failed live migration, which causes bad performance due to
disallowing huge page mapping.

Thanks to the previous patch, KVM can now stop hooking into
log_global_start/stop.  This simplifies the KVM code noticeably.

Reported-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Reported-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>