conf: move the check for secondary consoles of targetType serial

conf: move the check for secondary consoles of targetType serial

For historical reasons, only the first <console> element might be of targetType
serial, but we checked for other consoles of targetType serial in our post-parse
callback if and only if we knew the first console was serial, otherwise
the check was skipped.
This patch moves the check one level up, so first
the check for secondary console of type serial is performed and then the
rest of operations continue unchanged.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1170092