x86/svm: Reduce vmentry latency

x86/svm: Reduce vmentry latency

Writing to the stack pointer in the middle of a line of pop operations is
specifically recommended against by the optimisation guide, and is a technique
used by Speculative Load Hardening to combat SpectreRSB.

In practice, it causes all further stack-relative accesses to block until the
write to the stack pointer retires, so the stack engine can get back in sync.

Pop into any dead register to discard %rax's value without clobbering the
stack engine.  Smaller compiled code, and runs faster.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>