migration: Expose 'cancelling' status to user

'cancelling' status was introduced by commit 51cf4c1a, mainly to avoid a
possible start of a new migration process while the previous one still exists.
But we didn't expose this status to user, instead we returned the 'active' state.

Here, we expose it to the user (such as libvirt), 'cancelling' status only
occurs for a short window before the migration aborts, so for users,
if they cancel a migration process, it will observe 'cancelling' status
occasionally.

Testing revealed that with older libvirt (anything 1.2.13 or less) will
print an odd error message if the state is seen, but that the migration
is still properly cancelled. Newer libvirt will be patched to recognize
the new state without the odd error message.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Cc: libvir-list@redhat.com
Signed-off-by: Juan Quintela <quintela@redhat.com>
