x86: correctly check for sub-leaf zero of leaf 7 in pv_cpuid()

Only the low 32 bits are relevant.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -916,7 +916,7 @@ void pv_cpuid(struct cpu_user_regs *regs
         break;
 
     case 0x00000007:
-        if ( regs->ecx == 0 )
+        if ( regs->_ecx == 0 )
             b &= (cpufeat_mask(X86_FEATURE_BMI1) |
                   cpufeat_mask(X86_FEATURE_HLE)  |
                   cpufeat_mask(X86_FEATURE_AVX2) |