kvmtool: ARM: Use KVM_ARM_PREFERRED_TARGET vm ioctl to determine target cpu

Instead, of trying out each and every target type we should
use KVM_ARM_PREFERRED_TARGET vm ioctl to determine target type
for KVM ARM/ARM64.

If KVM_ARM_PREFERRED_TARGET vm ioctl fails then we fallback to
old method of trying all known target types.

If KVM_ARM_PREFERRED_TARGET vm ioctl succeeds but the returned
target type is not known to KVMTOOL then we forcefully init
VCPU with target type returned by KVM_ARM_PREFERRED_TARGET vm ioctl.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>