kvm tools, x86: Fix munmap() size in kvm__arch_delete_ram()

Sasha noticed during code review that commit e106906 ("kvm tool: Change
kvm->ram_size to real mapped size.") changed kvm->ram_size accounting to
include 32-bit memory hole but forgot to update munmap in
kvm__arch_delete_ram().

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>