This is useful in the next patch, to keep a saved copy
of vmcb12 registers and pass it around more easily.

Instead of blindly copying everything, we just copy EFER, CR0, CR3, CR4,
DR6 and DR7 which are needed by the VMRUN checks.  If more fields will
need to be checked, it will be quite obvious to see that they must be added
in struct vmcb_save_area_cached and in nested_copy_vmcb_save_to_cache().

__nested_copy_vmcb_save_to_cache() takes a vmcb_save_area_cached
parameter, which is useful in order to save the state to a local
variable.
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20211103140527.752797-3-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>