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: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
