block: Introduce bdrv_get_encrypted_filename (Jan Kiszka)

Introduce bdrv_get_encrypted_filename service to allow more informative
password prompting.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6704 c046a42c-6fe2-441c-8c8c-71466251a162
