KVM: Convert kvm_for_each_vcpu() to using xa_for_each_range()

Now that the vcpu array is backed by an xarray, use the optimised
iterator that matches the underlying data structure.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Message-Id: <20211116160403.4074052-8-maz@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>