util: don't support loopback and nbd when setuid

util: don't support loopback and nbd when setuid

The only remaining reason that virt-login-shell was trying to
link against virstoragefile was because of a call to
virStorageFileFormatTypeToString when spawning a qemu-nbd
process - but setuid processes shouldn't be spawning qemu-nbd.

* src/util/virfile.c (virFileLoopDeviceAssociate)
(virFileNBDDeviceAssociate): Cripple in setuid builds.
* src/Makefile.am (libvirt_setuid_rpc_client_la_SOURCES):
Drop virstoragefile from the list.

Signed-off-by: Eric Blake <eblake@redhat.com>