Description of problem:
  Block node is read-only when start a qcow2 image on https server and with auto-read-only=on set on its file node

Version-Release number of selected component (if applicable):
  qemu-kvm-rhev version:qemu-kvm-rhev-2.12.0-24.el7.x86_64
  kernel version:3.10.0-993.el7.x86_64

How reproducible:
  100%

Steps to Reproduce:
1.Create a qcow2 image on https server.
  （https server）#qemu-img create -f qcow2 base 1G

2.Start vm with a qcow2 image on https server with auto-read-only=on set on its file node.
  /usr/libexec/qemu-kvm \
-name 'avocado-vt-vm1' \
-machine pc  \
-nodefaults \
-device VGA,bus=pci.0,addr=0x2  \
-blockdev node-name=file_base,driver=https,sslverify=off,url=https://10.66.144.33/base,auto-read-only=on \
-blockdev driver=qcow2,file=file_base,node-name=base,auto-read-only=on \
-qmp tcp:0:3000,server,nowait \
-vnc :0 \
-monitor stdio \

Actual results:
  After step1, vm failed to start with qemu info:
    qemu-kvm: -blockdev driver=qcow2,file=file_base,node-name=base,auto-read-only=on: Block node is read-only


Expected results:
 VM can start successfully.

Additional info:
 1. When start vm with file node param "read-only=on", vm can start successfully.
  -blockdev node-name=file_base,driver=https,sslverify=off,url=https://10.66.144.33/base,auto-read-only=on \
  -blockdev driver=qcow2,file=file_base,node-name=base,read-only=on \

 2. With backend: localfs and qcow2 image + auto-read-only=on, vm can also start suceessfully.
  -blockdev node-name=file_base,driver=file,filename=/home/base,auto-read-only=on \
  -blockdev driver=qcow2,file=file_base,node-name=base,auto-read-only=on \