ArmVirtPkg: Kvmtool: Use Null version of DebugLib in PrePi

The patch at "6c8a08bd ArmVirtPkg/PrePi: Ensure timely
 execution of library constructors" moved the processing of
library constructors before the MMU is initialised.

This resulted in the BaseDebugLibSerialPort library constructor
BaseDebugLibSerialPortConstructor () which initialises the serial
port, being invoked before the MMU is enabled.

However, the Realm Code requires the protection attribute of
the MMIO regions to be configured as unprotected (shared with
the host), which requires the MMU to be enabled. Otherwise,
accesses to the MMIO region result in a synchronous external
abort being reflected to the Realm by the RMM.

Therefore, link the Null version of DebugLib in PrePi stage.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>