domain: add tsc.on_reboot element

domain: add tsc.on_reboot element

Some versions of Windows hang on reboot if their TSC value is greater
than 2^54.  The workaround is to reset the TSC to a small value.  Add
to the domain configuration an attribute for this.  It can be used
by QEMU and in principle also by ESXi, which has a property called
monitor_control.enable_softResetClearTSC as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>