qapi/monitor: refactor set/expire_password with enums

qapi/monitor: refactor set/expire_password with enums

'protocol' and 'connected' are better suited as enums than as strings,
make use of that. No functional change intended.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
[FE: update "Since: " from 6.2 to 7.0
     put 'keep' first in enum to ease use as a default]
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Message-Id: <20220225084949.35746-3-f.ebner@proxmox.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>