For some use-cases, it is helpful to have more than one UART
available to the guest.  If the second UART slot is not already used
for a TrustZone Secure-World-only UART, create it as a NonSecure UART
only when the user provides a serial backend (e.g.  via a second
-serial command line option).

This avoids problems where existing guest software only expects a
single UART, and gets confused by the second UART in the DTB.  The
major example of this is older EDK2 firmware, which will send the
GRUB bootloader output to UART1 and the guest serial output to UART0.
Users who want to use both UARTs with a guest setup including EDK2
are advised to update to EDK2 release edk2-stable202311 or newer.
(The prebuilt EDK2 blobs QEMU upstream provides are new enough.)
The relevant EDK2 changes are the ones described here:
https://bugzilla.tianocore.org/show_bug.cgi?id=4577

Inspired-by: Axel Heider <axel.heider@hensoldt.net>
Signed-off-by: Peter Maydell <peter.maydel...