storage: lvm: Don't overwrite lvcreate errors

Before:
$ sudo virsh vol-create-as --pool vgvirt sparsetest --capacity 16M --allocation 0
error: Failed to create vol sparsetest
error: internal error Child process (/usr/sbin/lvchange -aln vgvirt/sparsetest) unexpected exit status 5:   One or more specified logical volume(s) not found.

After:
$ sudo virsh vol-create-as --pool vgvirt sparsetest --capacity 16M --allocation 0
error: Failed to create vol sparsetest
error: internal error Child process (/usr/sbin/lvcreate --name sparsetest -L 0K --virtualsize 16384K vgvirt) unexpected exit status 5:   Unable to create new logical volume with no extents

src/storage/storage_backend_logical.c

@@ -699,6 +699,7 @@ virStorageBackendLogicalCreateVol(virConnectPtr conn,
 {
     int fdret, fd = -1;
     virCommandPtr cmd = NULL;
+    virErrorPtr err;
 
     if (vol->target.encryption != NULL) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
@@ -775,9 +776,11 @@ virStorageBackendLogicalCreateVol(virConnectPtr conn,
     return 0;
 
  cleanup:
+    err = virSaveLastError();
     VIR_FORCE_CLOSE(fd);
     virStorageBackendLogicalDeleteVol(conn, pool, vol, 0);
     virCommandFree(cmd);
+    virSetError(err);
     return -1;
 }