Allocate a shadow vmcs used by the processor to shadow part of the fields
stored in the software defined VMCS12 (let L1 access fields without causing
exits). Note we keep a shadow vmcs only for the current vmcs12. Once a vmcs12
becomes non-current, its shadow vmcs is released.
Signed-off-by: Abel Gordon <abelg@il.ibm.com> Reviewed-by: Orit Wasserman <owasserm@redhat.com> Signed-off-by: Gleb Natapov <gleb@redhat.com>