Recently e1000 NIC support was added to bhyve; implement that in
the bhyve driver:

 - Add capability check by analyzing output of the 'bhyve -s 0,e1000'
   command
 - Modify bhyveBuildNetArgStr() to support e1000 and also pass
   virConnectPtr so it could call bhyveDriverGetCaps() to check if this
   NIC is supported
 - Modify command parsing code to add support for e1000 and adjust tests
 - Add net-e1000 test