qemu: catch cdrom change error

qemu: catch cdrom change error

Currently when we attempt to change the cdrom in a qemu VM the monitor
doesn't generate an error if the target filename doesn't exist.  I've
submitted a patch[1] for this.  This patch is the libvirt qemu-driver
side which catches the error message from the monitor and reportes the
error to libvirt.  This means that virsh attach-disk cdrom commands
won't appear to succeed when qemu change command actually failed.

* src/qemu/qemu_monitor_text.c: in qemuMonitorTextChangeMedia() look
  for failure to access the new data