autobuild.sh: provide default prefix
autobuild.sh: provide default prefix

Without this patch and with a clean environment, ./autobuild.sh
tried to use ./configure --prefix=/, and fails.

* autobuild.sh (AUTOBUILD_INSTALL_ROOT): Provide sensible
default.  Suggested by Daniel P. Berrange.

amery/libvirt-vserver,foomango/libvirt,kantai/libvirt-vfork,leilihh/libvirt,jeckersb/libvirt,wiedi/libvirt,novel/fbsd-libvirt,rbu/libvirt,bjzhang/libvirt,elmarco/libvirt,warewolf/libvirt,rmarwaha/libvirt,taget/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,nertpinx/libvirt,rmarwaha/libvirt1,rmarwaha/libvirt1,jardasgit/libvirt,soulxu/libvirt-xuhj,jardasgit/libvirt,rlaager/libvirt,agx/libvirt,siboulet/libvirt-openvz,jardasgit/libvirt,jeckersb/libvirt,amery/libvirt-vserver,danwent/libvirt-ovs,datto/libvirt,eskultety/libvirt,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,danwent/libvirt-ovs,rmarwaha/libvirt1,eskultety/libvirt,wiedi/libvirt,jfehlig/libvirt,agx/libvirt,jeckersb/libvirt,novel/fbsd-libvirt,cbosdo/libvirt,trainstack/libvirt,crobinso/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,danwent/libvirt-ovs,warewolf/libvirt,warewolf/libvirt,siboulet/libvirt-openvz,novel/fbsd-libvirt,taget/libvirt,wiedi/libvirt,usc-isi/libvirt,warewolf/libvirt,cbosdo/libvirt,iam-TJ/libvirt,cbosdo/libvirt,wiedi/libvirt,olafhering/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,siboulet/libvirt-openvz,bjzhang/libvirt,datto/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,siboulet/libvirt-openvz,iam-TJ/libvirt,zhlcindy/libvirt-1.1.4-maintain,rmarwaha/libvirt,VenkatDatta/libvirt,leilihh/libvirt,kantai/libvirt-vfork,elmarco/libvirt,agx/libvirt,jfehlig/libvirt,bjzhang/libvirt,elmarco/libvirt,kantai/libvirt-vfork,soulxu/libvirt-xuhj,leilihh/libvirt,foomango/libvirt,VenkatDatta/libvirt,rmarwaha/libvirt,leilihh/libvirt,amery/libvirt-vserver,novel/fbsd-libvirt,rbu/libvirt,novel/fbsd-libvirt,rbu/libvirt,shugaoye/libvirt,taget/libvirt,novel/fbsd-libvirt,trainstack/libvirt,foomango/libvirt,rbu/libvirt,jardasgit/libvirt,trainstack/libvirt,zippy2/libvirt,eskultety/libvirt,fabianfreyer/libvirt,usc-isi/libvirt,libvirt/libvirt,warewolf/libvirt,jfehlig/libvirt,zhlcindy/libvirt-1.1.4-maintain,olafhering/libvirt,elmarco/libvirt,rmarwaha/libvirt,amery/libvirt-vserver,leilihh/libvirt,trainstack/libvirt,rlaager/libvirt,dumbbell/libvirt,iam-TJ/libvirt,VenkatDatta/libvirt,eskultety/libvirt,fabianfreyer/libvirt,rmarwaha/libvirt1,zhlcindy/libvirt-1.1.4-maintain,zhlcindy/libvirt-1.1.4-maintain,wiedi/libvirt,jfehlig/libvirt,rbu/libvirt,crobinso/libvirt,elmarco/libvirt,amery/libvirt-vserver,eskultety/libvirt,taget/libvirt,iam-TJ/libvirt,jeckersb/libvirt,shugaoye/libvirt,jeckersb/libvirt,shugaoye/libvirt,dumbbell/libvirt,olafhering/libvirt,emaste/libvirt,rlaager/libvirt,trainstack/libvirt,dumbbell/libvirt,andreabolognani/libvirt,libvirt/libvirt,foomango/libvirt,siboulet/libvirt-openvz,danwent/libvirt-ovs,emaste/libvirt,nertpinx/libvirt,kantai/libvirt-vfork,wiedi/libvirt,dumbbell/libvirt,VenkatDatta/libvirt,emaste/libvirt,crobinso/libvirt,iam-TJ/libvirt,wiedi/libvirt,zippy2/libvirt,emaste/libvirt,rlaager/libvirt,warewolf/libvirt,usc-isi/libvirt,taget/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,jeckersb/libvirt,libvirt/libvirt,soulxu/libvirt-xuhj,emaste/libvirt,datto/libvirt,bjzhang/libvirt,leilihh/libvirt,jeckersb/libvirt,agx/libvirt,crobinso/libvirt,trainstack/libvirt,rlaager/libvirt,datto/libvirt,dumbbell/libvirt,soulxu/libvirt-xuhj,bjzhang/libvirt,rmarwaha/libvirt,cbosdo/libvirt,zhlcindy/libvirt-1.1.4-maintain,novel/fbsd-libvirt,usc-isi/libvirt,andreabolognani/libvirt,novel/fbsd-libvirt,rmarwaha/libvirt1,cbosdo/libvirt,fabianfreyer/libvirt,foomango/libvirt,datto/libvirt,dumbbell/libvirt,emaste/libvirt,libvirt/libvirt,warewolf/libvirt,fabianfreyer/libvirt,iam-TJ/libvirt,agx/libvirt,emaste/libvirt,shugaoye/libvirt,jardasgit/libvirt,rmarwaha/libvirt1,kantai/libvirt-vfork,andreabolognani/libvirt,nertpinx/libvirt,rmarwaha/libvirt,shugaoye/libvirt,trainstack/libvirt,iam-TJ/libvirt,andreabolognani/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,usc-isi/libvirt,sshah-solarflare/Libvirt-PCI-passthrough-,novel/fbsd-libvirt,zippy2/libvirt,soulxu/libvirt-xuhj,danwent/libvirt-ovs