monitor: do_cont(): Don't ask for passwords

The do_cont() function will ask the user to enter a password if a
device is encrypted.

This is invalid under QMP, so we raise a QERR_DEVICE_ENCRYPTED
error.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
