conf: relocate virDomainDeviceDef and virDomainHostdevDef

conf: relocate virDomainDeviceDef and virDomainHostdevDef

This patch is only code movement + adding some forward definitions of
typedefs.

virDomainHostdevDef (not just a pointer to it, but an actual object)
will be needed in virDomainNetDef and virDomainActualNetDef, so it
must be relocated earlier in the file.

Likewise, virDomainDeviceDef will be needed in virDomainHostdevDef, so
it must be moved up even earlier. This, in turn, creates a forward
reference problem, but fortunately only with pointers to other device
types, so their typedefs can be moved up in the file, eliminating the
problem.