xenconfig: Properly check retval of virDomainGraphicsListenSetAddress
xenconfig: Properly check retval of virDomainGraphicsListenSetAddress

The function, like others in our code, returns zero on success
and a negative value on error. However, there are two places in
xenconfig source code where we check for non-zero value. While
the function can't currently return a positive value, those
checks look okay, but does not really follow our style.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

jardasgit/libvirt,jardasgit/libvirt,taget/libvirt,taget/libvirt,andreabolognani/libvirt,nertpinx/libvirt,datto/libvirt,eskultety/libvirt,zippy2/libvirt,datto/libvirt,zippy2/libvirt,rlaager/libvirt,rlaager/libvirt,rlaager/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,datto/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,andreabolognani/libvirt,rlaager/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt,taget/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,eskultety/libvirt,VenkatDatta/libvirt,olafhering/libvirt,jardasgit/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,crobinso/libvirt,datto/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,jardasgit/libvirt,eskultety/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,datto/libvirt,eskultety/libvirt,eskultety/libvirt,crobinso/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,libvirt/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,olafhering/libvirt,fabianfreyer/libvirt,rlaager/libvirt,taget/libvirt,crobinso/libvirt,fabianfreyer/libvirt,taget/libvirt,jfehlig/libvirt