exec: Do vmstate unregistration from cpu_exec_exit()

cpu_exec_init() does vmstate_register for the CPU device. This needs to be
undone from cpu_exec_exit(). This change is needed to support CPU hot
removal.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
[dwg: added missing include to fix compile on some archs]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>