bhyve: monitor: do not override domain's privateData

bhyve: monitor: do not override domain's privateData

Current monitor code overrides domain object's privateData, e.g.
in virBhyveProcessStart():

  vm->privateData = bhyveMonitorOpen(vm, driver);

where bhyveMonitorPtr() returns bhyveMonitorPtr.

This is not right thing to do, so make bhyveMonitorPtr
a part of the bhyveDomainObjPrivate struct and change related code
accordingly.