kvm: x86: add tsc_offset field to struct kvm_vcpu_arch

A future commit will want to easily read a vCPU's TSC offset,
so we store it in struct kvm_arch_vcpu_arch for easy access.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>