block: expose crypto option names / defs to other drivers

The block/crypto.c defines a set of QemuOpts that provide
parameters for encryption. This will also be needed by
the qcow/qcow2 integration, so expose the relevant pieces
in a new block/crypto.h header. Some helper methods taking
QemuOpts are changed to take QDict to simplify usage in
other places.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-2-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
