storage: Ensure the device path exists before refreshing disk pool
storage: Ensure the device path exists before refreshing disk pool

Doing libvirt_parthelper on an not existed device path will get
an unfriendly error message. This patch is to prohibit it.

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