hyperv/kvp: wake up the daemon if it's sleeping due to poll()

Without the patch, there is a race condition: when poll() is invoked(),
if kvp_globals.daemon_busy is false, the daemon won't be timely
woke up, because hv_kvp_send_msg_to_daemon() can't wake up the daemon
in this case.

Submitted by: Dexuan Cui <decui@microsoft.com>
Sponsored by: Microsoft OSTC
Reviewed by: delphij, royger
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D4258