libvirt: Fix crash on URI without scheme
libvirt: Fix crash on URI without scheme

My commit 0d1579572 crashes on a URI without a scheme, like via
'virsh --connect frob'

Add a check on uri->server too while we are at it, and centralize
them all

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