src/virt.c: add lifecycle transition notifications

src/virt.c: add lifecycle transition notifications

Notifications about VM state are received on every read interval, so the same
notification about specific VM state is received several times within a minute,
which lead to growing log files. This issue is fixed by generating notifications
only for VM state changes (denoted also here lifecycle transitions).

This commit adds a binary option PersistentNotification to virt plugin. When
this option is set to false (default value), only VM state changes are notified
as soon as they occur. When this option is set to true, VM states are notified
on every read interval.

Change-Id: I46ebe2d2f1b72432e7b458530c4cc0eac3e26c6a
Signed-off-by: Antoine Naud <antoinex.naud@intel.com>