KVM: arm64: Replace vCPU target with a configuration flag

The value of kvm_vcpu_arch::target has been used to determine if a vCPU
has actually been initialized. Storing this as an integer is needless at
this point, as KVM doesn't do any microarch-specific emulation in the
first place. Instead, all we care about is whether or not the vCPU has
been initialized.

Delete the field in favor of a vCPU configuration flag indicating if
KVM_ARM_VCPU_INIT has completed for the vCPU.

Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230710193140.1706399-4-oliver.upton@linux.dev