hmp: hmp_cont(): don't rely on QERR_DEVICE_ENCRYPTED

This commit changes hmp_cont() to loop through all block devices
and proactively set an encryption key for any encrypted device
missing a key.

This change is needed because QERR_DEVICE_ENCRYPTED is going to be
dropped by a future commit.

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