virDomainDefAddConsoleCompat: Fix numbering of console targets after modification

virDomainDefAddConsoleCompat: Fix numbering of console targets after modification

The XML parser for consoles sets the 'port=' attribute of '<target' to
be always the index of the console.

Thus when the "really crazy backcompat stuff for consoles" function
modifies the order of consoles by inserting the default one for a serial
port it must re-number the ports to ensure that the value will not
change on subsequent parse.

This luckily didn't cause any visible changes to the VM as the port
number isn't used for anything.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>