bhyve: fix virObjectUnlock() usage

bhyve: fix virObjectUnlock() usage

In a number of places in the bhyve driver, virObjectUnlock()
is called with an arg without check if the arg is non-NULL, which
could result in passing NULL value and a warning like:

virObjectUnlock:340 : Object 0x0 ((unknown)) is not a virObjectLockable instance

* src/bhyve/bhyve_driver.c (bhyveDomainGetInfo)
(bhyveDomainGetState, bhyveDomainGetAutostart)
(bhyveDomainSetAutostart, bhyveDomainIsActive)
(bhyveDomainIsPersistent, bhyveDomainGetXMLDesc)
(bhyveDomainUndefine, bhyveDomainLookupByUUID)
(bhyveDomainLookupByName, bhyveDomainLookupByID)
(bhyveDomainCreateWithFlags, bhyveDomainOpenConsole):
Check if arg is not NULL before calling virObjectUnlock on it.