unify popen/fopen qemu wrappers

While reading Chris's code for fd migration I noticed the duplication
between QEMUFilePopen and QEMUFileStdio.  This fixes it, and makes
qemu_fopen more similar qemu_popen.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/migration-exec.c b/migration-exec.c
index ef4620f..b45c833 100644
--- a/migration-exec.c
+++ b/migration-exec.c
@@ -114,7 +114,7 @@
     qemu_announce_self();
     dprintf("successfully loaded vm state\n");
     /* we've successfully migrated, close the fd */
-    qemu_set_fd_handler2(qemu_popen_fd(f), NULL, NULL, NULL, NULL);
+    qemu_set_fd_handler2(qemu_stdio_fd(f), NULL, NULL, NULL, NULL);
     if (autostart)
         vm_start();
 
@@ -133,7 +133,7 @@
         return -errno;
     }
 
-    qemu_set_fd_handler2(qemu_popen_fd(f), NULL,
+    qemu_set_fd_handler2(qemu_stdio_fd(f), NULL,
 			 exec_accept_incoming_migration, NULL,
 			 (void *)(unsigned long)f);