hmp_change(): don't access DeviceEncrypted's data

It's not needed. As the device name is already known, we can replace
the duplicated password prompting code by monitor_read_block_device_key().

This overly simplifies hmp_change().

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
