arm64: Create a realm virtual machine

Set the machine type to realm when creating a VM via the KVM_CREATE_VM
ioctl.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
[ Alex E: Reworked patch, split the command line option into a different
          patch ]
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
[ Suzuki: Exit early if Realm support is not available ]
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>