migration/ram.c: Fix codes conflict about bitmap_mutex

migration/ram.c: Fix codes conflict about bitmap_mutex

I found upstream codes conflict with COLO and lead to crash,
and I located to this patch:

commit 386a907b37a9321bc5d699bc37104d6ffba1b34d
Author: Wei Wang <wei.w.wang@intel.com>
Date:   Tue Dec 11 16:24:49 2018 +0800

migration: use bitmap_mutex in migration_bitmap_clear_dirty

My colleague Wei's patch add bitmap_mutex in migration_bitmap_clear_dirty,
but COLO didn't initialize the bitmap_mutex. So we always get an error
when COLO start up. like that:
qemu-system-x86_64: util/qemu-thread-posix.c:64: qemu_mutex_lock_impl: Assertion `mutex->initialized' failed.

This patch add the bitmap_mutex initialize and destroy in COLO
lifecycle.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Message-Id: <20190329222951.28945-1-chen.zhang@intel.com>
Reviewed-by: Wei Wang <wei.w.wang@intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>