plat/kvm/shutdown.c: If on a `UEFI` system, rely on Runtime Services

plat/kvm/shutdown.c: If on a `UEFI` system, rely on Runtime Services

Preferably, a UEFI system is also an ACPI system and thus these
functionalities should be implemented through ACPI methods. But for
now, make use of UEFI's Runtime Services for resetting the system,
since they are more reliable than what we have at the moment.

Signed-off-by: Sergiu Moga <sergiu.moga@protonmail.com>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #909