bhyve: implement domainShutdown

bhyve: implement domainShutdown

Bhyve supports ACPI shutdown by issuing SIGTERM signal to a bhyve
process.

Add the bhyveDomainShutdown() function and virBhyveProcessShutdown()
helper function that just sends SIGTERM to VM's bhyve process. If
a guest supports ACPI shutdown then process will be terminated and
this event will be noticed by the bhyve monitor code that will handle
setting proper status and clean up VM's resources by calling
virBhyveProcessStop().