x86/SVM: correct boot time cpu_data[] handling

x86/SVM: correct boot time cpu_data[] handling

start_svm() already runs after cpu_data[] was set up, so it shouldn't
modify it anymore (at least not directly). Constify the involved
pointers.

Furthermore LMSLE feature detection was broken by 566ddbe833 ("x86:
Fail CPU bringup cleanly if it cannot initialise HVM"), as Andrew
Cooper has pointed out: c couldn't possibly equal &boot_cpu_data
anymore. (But since it's unsafe migration-wise for some more time,
suppress the feature actually being enabled for us.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>