KVM: selftests: Move XGETBV and XSETBV helpers to common code

The instructions XGETBV and XSETBV are useful to other tests.  Move
them to processor.h to make them more broadly available.

No functional change intended.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
[sean: reword shortlog]
Reviewed-by: Aaron Lewis <aaronlewis@google.com>
Tested-by: Aaron Lewis <aaronlewis@google.com>
Link: https://lore.kernel.org/r/20230405004520.421768-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>