bhyve: drop virProcessKillPainfully() from destroy

bhyve: drop virProcessKillPainfully() from destroy

Current implementation of domainDestroy for bhyve calls
virProcessKillPainfully() for the bhyve process and then
executes "bhyvectl --destroy".

This is wrong for two reasons:

 * bhyvectl --destroy alone is sufficient because it terminates
   the process
 * virProcessKillPainfully() first sends SIGTERM and after few
   attempts sends SIGKILL. As SIGTERM triggers ACPI shutdown that
   we're not interested in, it creates an unwanted side effect in
   domainDestroy.

Also, destroy monitor only after "bhyvectl --destroy" command succeeded
to avoid a case when the command fails and domain remains running, but
not being monitored anymore.