qemu: command: Split out network disk URI building
qemu: command: Split out network disk URI building

Extract the code so that it can be called from multiple places. This
also removes a tricky fallthrough in the large switch in
qemuBuildNetworkDriveStr.

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