x86/HVM: memset CPU context save area

x86/HVM: memset CPU context save area

In order to prevent leaking data in the padding field. Also remove the
memset done to the fpu_regs in case of no FPU context present, since it's
already taken care by the memset of the whole CPU context structure. The
same applies to setting ctxt.flags to 0 in case there's no FPU context.

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