Support 'block_passwd' command for QEMU disk encryption

Support 'block_passwd' command for QEMU disk encryption

The old text mode monitor prompts for a password when disks are
encrypted. This interactive approach doesn't work for JSON mode
monitor. Thus there is a new 'block_passwd' command that can be
used.

* src/qemu/qemu_driver.c: Split out code for looking up a disk
  secret from findVolumeQcowPassphrase, into a new method
  getVolumeQcowPassphrase. Enhance qemuInitPasswords() to also
  set the disk encryption password via the monitor
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
  src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h,
  src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h: Add
  support for the 'block_passwd' monitor command.