x86 svm: Make 32bit legacy guests boot again

x86 svm: Make 32bit legacy guests boot again

Attached patch fixes a bug introduced in c/s 19648.

32bit legacy guests have the sysenter/sysexit instructions available.
Therefore, we have to disable intercepts for the sysenter MSRs or the
guest stucks in an infinite loop of #GPs, otherwise.

For guests in 64bit mode and 32bit compat mode, sysenter/sysexit
instructions aren't available. The sysenter MSRs have to be
intercepted to make the instruction emulation working.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>