Status change reason support for Reboot

Add support to the Reboot command for specifying the reason for the last
status change.

Some features are implemented as functions, even if used only once, because
they will be used by the future patches introducing reason support for all
the others commands able to alter the state of instances.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>