Improve error reporting for virConnectGetHostname calls

All drivers have copy + pasted inadequate error reporting which wraps
util.c:virGetHostname. Move all error reporting to this function, and improve
what we report.

Changes from v1:
  Drop the driver wrappers around virGetHostname. This means we still need
  to keep the new conn argument to virGetHostname, but I think it's worth
  it.

tools/virsh.c

@@ -524,7 +524,7 @@ cmdRunConsole(vshControl *ctl, virDomainPtr dom)
     char *thatHost = NULL;
     char *thisHost = NULL;
 
-    if (!(thisHost = virGetHostname())) {
+    if (!(thisHost = virGetHostname(ctl->conn))) {
         vshError(ctl, "%s", _("Failed to get local hostname"));
         goto cleanup;
     }