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 <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
[dwg: added missing include to fix compile on some archs]
Signed-off-by: David Gibson <[email protected]>