target-i386: Defer MCE init

Commit de024815e3b523addf58f1f79846b7fe74643678 (target-i386: QOM'ify
CPU init) moved mce_init() call from helper.c:cpu_x86_init() into
X86CPU's cpu.c:x86_cpu_initfn().
mce_init() checks for a family >= 6 though, so we could end up with a
sequence such as for -cpu somecpu,family=6:

  x86_cpu_initfn => X86CPU::family == 5
    mce_init => no-op
  cpu_x86_register => X86CPU::family = 6
  => MCE unexpectedly not init'ed

or for -cpu someothercpu,family=5:

  x86_cpu_initfn => X86CPU::family == 6
    mce_init => init'ed
  cpu_x86_register => X86CPU::family = 5
  => MCE unexpectedly init'ed

Therefore partially revert the above commit. To avoid moving
mce_init() back into helper.c, foresightedly move it into a
new x86_cpu_realize() function and, in lack of ObjectClass::realize,
call it directly from cpu_x86_init().

While at it, move the qemu_init_vcpu() call that used to follow
mce_init() in cpu_x86_init() into the new realizefn as well.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
