Move encryption lookup back into qemu driver file

Move encryption lookup back into qemu driver file

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.