Allow qemu driver (and hence libvirtd) to load when qemu
user:group does not exist.  The kvm package, which may not
exist on a xen host, creates qemu user:group.

A better (future) solution would be to build the libvirtd
drivers as loadable modules instead of built-in to the
daemon.  Then the qemu driver would only be loaded when needed,
which would never be the case on a xen-only configuration.

Index: libvirt-0.9.13/src/qemu/qemu_conf.c
===================================================================
--- libvirt-0.9.13.orig/src/qemu/qemu_conf.c
+++ libvirt-0.9.13/src/qemu/qemu_conf.c
@@ -271,9 +271,7 @@ int qemudLoadDriverConfig(struct qemud_d
         return -1;
     }
     if (virGetUserID(user, &driver->user) < 0) {
-        VIR_FREE(user);
-        virConfFree(conf);
-        return -1;
+        VIR_WARN("User %s does not exist! Continuing...", user);
     }
     VIR_FREE(user);
 
@@ -286,9 +284,7 @@ int qemudLoadDriverConfig(struct qemud_d
         return -1;
     }
     if (virGetGroupID(group, &driver->group) < 0) {
-        VIR_FREE(group);
-        virConfFree(conf);
-        return -1;
+        VIR_WARN("Group %s does not exist! Continuing...", group);
     }
     VIR_FREE(group);