Decouple the monitor code from the virDomainDefPtr structure
by moving the disk encryption lookup code back into the
qemu_driver.c file. Instead provide a function callback to
the monitor code which can be invoked to retrieve encryption
data as required.

* src/qemu/qemu_driver.c: Add findDomainDiskEncryption,
  and findVolumeQcowPassphrase. Pass address of the method
  findVolumeQcowPassphrase into qemuMonitorOpen()
* src/qemu/qemu_monitor.c: Associate a disk
  encryption function callback with the qemuMonitorPtr
  object.
* src/qemu/qemu_monitor_text.c: Remove findDomainDiskEncryption
  and findVolumeQcowPassphrase.