spec: proper soft static allocation of qemu uid

https://bugzilla.redhat.com/show_bug.cgi?id=924501 tracks a
problem that occurs if uid 107 is already in use at the time
libvirt is first installed.  In response that problem, Fedora
packaging guidelines were recently updated.  This fixes the
spec file to comply with the new guidelines:
https://fedoraproject.org/wiki/Packaging:UsersAndGroups

* libvirt.spec.in (daemon): Follow updated Fedora guidelines.

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

libvirt.spec.in

@@ -718,6 +718,8 @@ Requires: numad
     %endif
 # libvirtd depends on 'messagebus' service
 Requires: dbus
+# For uid creation during pre
+Requires(pre): shadow-utils
 
 %description daemon
 Server side daemon required to manage the virtualization capabilities
@@ -1444,14 +1446,19 @@ make check
 %if %{with_libvirtd}
 %pre daemon
     %if 0%{?fedora} >= 12 || 0%{?rhel} >= 6
-# Normally 'setup' adds this in /etc/passwd, but this is
-# here for case of upgrades from earlier Fedora/RHEL. This
-# UID/GID pair is reserved for qemu:qemu
-getent group kvm >/dev/null || groupadd -g 36 -r kvm
-getent group qemu >/dev/null || groupadd -g 107 -r qemu
-getent passwd qemu >/dev/null || \
-  useradd -r -u 107 -g qemu -G kvm -d / -s /sbin/nologin \
-    -c "qemu user" qemu
+# We want soft static allocation of well-known ids, as disk images
+# are commonly shared across NFS mounts by id rather than name; see
+# https://fedoraproject.org/wiki/Packaging:UsersAndGroups
+getent group kvm >/dev/null || groupadd -f -g 36 -r kvm
+getent group qemu >/dev/null || groupadd -f -g 107 -r qemu
+if ! getent passwd qemu >/dev/null; then
+  if ! getent passwd 107 >/dev/null; then
+    useradd -r -u 107 -g qemu -G kvm -d / -s /sbin/nologin -c "qemu user" qemu
+  else
+    useradd -r -g qemu -G kvm -d / -s /sbin/nologin -c "qemu user" qemu
+  fi
+fi
+exit 0
     %endif
 
 %post daemon