KVM: Introduce kvm_arch_destroy_vcpu()

KVM: Introduce kvm_arch_destroy_vcpu()

Simiar to how kvm_init_vcpu() calls kvm_arch_init_vcpu() to perform
arch-dependent initialisation, introduce kvm_arch_destroy_vcpu()
to be called from kvm_destroy_vcpu() to perform arch-dependent
destruction.

This was added because some architectures (Such as i386)
currently do not free memory that it have allocated in
kvm_arch_init_vcpu().

Suggested-by: Maran Wilson <maran.wilson@oracle.com>
Reviewed-by: Maran Wilson <maran.wilson@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Message-Id: <20190619162140.133674-3-liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>