SVM: introduce a VM entry helper

SVM: introduce a VM entry helper

Neither the register values copying nor the trace entry generation need
doing in assembly. The VMLOAD invocation can also be further deferred
(and centralized). Therefore replace the svm_asid_handle_vmrun()
invocation with one of the new helper.

Similarly move the VM exit side register value copying into
svm_vmexit_handler().

Now that we always make it out to guest context after VMLOAD,
svm_sync_vmcb() no longer overrides vmcb_needs_vmsave, making
svm_vmexit_handler() setting the field early unnecessary.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Release-acked-by: Juergen Gross <jgross@suse.com>