target-i386: clear guest TSC on reset

VCPU TSC is not cleared by a warm reset (*), which leaves some types of Linux
 guests (non-pvops guests and those with the kernel parameter no-kvmclock set)
vulnerable to the overflow in cyc2ns_offset fixed by upstream commit
9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 ("sched/x86: Fix overflow in
cyc2ns_offset").

To put it in a nutshell, if such a Linux guest without the patch above applied
has been up more than 208 days and attempts a warm reset chances are that
the newly booted kernel will panic or hang.

(*) Intel Xeon E5 processors show the same broken behavior due to
    the errata "TSC is Not Affected by Warm Reset" (Intel® Xeon®
    Processor E5 Family Specification Update - August 2013): "The
    TSC (Time Stamp Counter MSR 10H) should be cleared on
    reset. Due to this erratum the TSC is not affected by warm
    reset."

Cc: Will Auld <will.auld@intel.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fernando Luis Vázquez Cao <fernando_b1@lab.ntt.co.jp>
