qemu: domain: Reject copy_on_read for read-only disks

qemu: domain: Reject copy_on_read for read-only disks

The sectors read from the backing image need to be written to the top
level image. If a disk is marked read-only the image can't be written.

QEMU handled that by disabling copy_on_read and reporting a warning:

-drive file=/var/lib/libvirt/images/c,format=qcow2,if=none,
    id=drive-scsi0-0-1,readonly=on,copy-on-read=on:
    warning: disabling copy-on-read on read-only drive

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>