hostmem: Fix mem-path property name in error report

The subtle difference between "property not found" and "property not
set" is already confusing enough.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

backends/hostmem-file.c

@@ -43,7 +43,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         return;
     }
     if (!fb->mem_path) {
-        error_setg(errp, "mem_path property not set");
+        error_setg(errp, "mem-path property not set");
         return;
     }
 #ifndef CONFIG_LINUX