Add new netlink events helpers tcmu_netlink_event_init() and
tcmu_netlink_event_send(). These new functions intend to replace
existing netlink events helper function tcmu_netlink_event().

The existing function tcmu_netlink_event() works well for events like
TCMU_ADDED_DEVICE and TCMU_REMOVED_DEVICE which only has one netlink
attribute. But if there is a command requires more than one attributes
to send out, we have to use a struct to adapt the paremeter
reconfig_data, it is hard to use one struct or a union in one struct to
adapt every command with different attributes, it may get long and ugly.

With the new two functions, we can call tcmu_netlink_event_init() to
initialize a netlink event, then add all attributes we need by using
nla_put_xxx(), at last use tcmu_netlink_event_send() to send it out. So
that we don't need to use a long struct or union if we want to send
mulitple attributes for different commands.

[mkp: typos]
Signed-off-by: Zhu Lingshan <lszhu@suse.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>