KVM: VMX: Initialize vm86 TSS only once.

Currently vm86 task is initialized on each real mode entry and vcpu
reset. Initialization is done by zeroing TSS and updating relevant
fields. But since all vcpus are using the same TSS there is a race where
one vcpu may use TSS while other vcpu is initializing it, so the vcpu
that uses TSS will see wrong TSS content and will behave incorrectly.
Fix that by initializing TSS only once.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>