hmp: Add flag for preconfig commands

hmp: Add flag for preconfig commands

Add a flag to command definitions to allow them to be used in preconfig
and check it.
If users try to use commands that aren't available, tell them to use
the exit_preconfig comand we're adding in a few patches.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20180620153947.30834-2-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>