target-i386: move cpu_reset and reset callback to cpu.c

Moving reset callback into cpu object from board level and
resetting cpu at the end of x86_cpu_realize() will allow properly
create cpu object during run-time (hotplug) without calling reset externaly.

When reset over QOM hierarchy is implemented, reset callback
should be removed.

v2:
  - leave cpu_reset in pc_new_cpu() for now, it's to be cleaned up when APIC
    init is moved in cpu.c

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>