hv: switch to cpuhp state machine for synic init/cleanup

hv: switch to cpuhp state machine for synic init/cleanup

BugLink: http://bugs.launchpad.net/bugs/1676635
To make it possible to online/offline CPUs switch to cpuhp infrastructure
for doing hv_synic_init()/hv_synic_cleanup().

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 76d36ab79820430f73c584673aef10ba2446fced)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>