ui: Check numeric part of expire_password argument @time properly

ui: Check numeric part of expire_password argument @time properly

When argument @time isn't 'now' or 'never', we parse it as an integer,
optionally prefixed with '+'.  If parsing fails, we silently assume
zero.  Report an error and fail instead.

While there, use qemu_strtou64() instead of strtoull() so
checkpatch.pl won't complain.

Aside: encoding numbers in strings is bad QMP practice.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230109190321.1056914-2-armbru@redhat.com>