qemu: Fix hot (un-)plug error codes and messages
qemu: Fix hot (un-)plug error codes and messages

With current code, error reporting for unsupported devices for hot plug,
unplug and update is total mess. The VIR_ERR_CONFIG_UNSUPPORTED error
code is reported instead of VIR_ERR_OPERATION_UNSUPPORTED. Moreover, the
error messages are not helping to find the root cause (lack of
implementation).

cbosdo/libvirt,eskultety/libvirt,libvirt/libvirt,agx/libvirt,andreabolognani/libvirt,olafhering/libvirt,fabianfreyer/libvirt,crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,elmarco/libvirt,taget/libvirt,shugaoye/libvirt,cbosdo/libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,taget/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,nertpinx/libvirt,jfehlig/libvirt,taget/libvirt,nertpinx/libvirt,rlaager/libvirt,jardasgit/libvirt,eskultety/libvirt,andreabolognani/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,rlaager/libvirt,agx/libvirt,jardasgit/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,datto/libvirt,olafhering/libvirt,datto/libvirt,agx/libvirt,agx/libvirt,elmarco/libvirt,libvirt/libvirt,eskultety/libvirt,VenkatDatta/libvirt,rlaager/libvirt,crobinso/libvirt,taget/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,libvirt/libvirt,andreabolognani/libvirt,elmarco/libvirt,jfehlig/libvirt,eskultety/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,elmarco/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,zippy2/libvirt,zippy2/libvirt,VenkatDatta/libvirt,elmarco/libvirt,jfehlig/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,rlaager/libvirt,agx/libvirt,cbosdo/libvirt,datto/libvirt,crobinso/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt,jardasgit/libvirt,datto/libvirt,rlaager/libvirt,eskultety/libvirt,VenkatDatta/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,libvirt/libvirt,shugaoye/libvirt,taget/libvirt,andreabolognani/libvirt,nertpinx/libvirt