target-i386: avoid overflow in the tsc-frequency property

The TSC frequency fits comfortably in an int when expressed in kHz,
but it may overflow when converted to Hz.  In this case,
tsc-frequency returns a negative value because x86_cpuid_get_tsc_freq
does a 32-bit multiplication before assigning to int64_t.

For simplicity just make tsc_khz a 64-bit value.

Spotted by Coverity.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

target-i386/cpu.h

@@ -962,7 +962,7 @@ typedef struct CPUX86State {
     uint8_t has_error_code;
     uint32_t sipi_vector;
     bool tsc_valid;
-    int tsc_khz;
+    int64_t tsc_khz;
     void *kvm_xsave_buf;
 
     uint64_t mcg_cap;