SMBIOS: followup: allow now 512 characters for full format string

to acutally allow to set the values requested by #2190 we need to
alter the limit a bit, as else the requested values cannot be save.

Just double it to 512, which should be really enough for this, else
go complain to your software vendor for needing such _hacks_ in the
first place..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

PVE/QemuServer.pm

@@ -592,7 +592,7 @@ EODESCR
     smbios1 => {
 	description => "Specify SMBIOS type 1 fields.",
 	type => 'string', format => 'pve-qm-smbios1',
-	maxLength => 256,
+	maxLength => 512,
 	optional => 1,
     },
     protection => {