x86/cpuid: Tighten parsing of tsc_freq=FREQ

cpu_x86_find_by_name() uses strtosz_suffix_unit(), but screws up the
error checking.  It detects some failures, but not all.  Undetected
failures result in a zero tsc_khz value (error value -1 divided by
1000), which means "no tsc_freq set".

To reproduce, try "-cpu qemu64,tsc_freq=9999999T".
strtosz_suffix_unit() fails, because the value overflows int64_t,

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
