xm,xend: Add new option "change_home_server" to xm migrate

xm,xend: Add new option "change_home_server" to xm migrate

This patch adds a new option to xm migrate command.  A concept of the
option is inspired from XenServer/XenCenter.  The concept is "Change
home server (affinity)."  The option name is "change_home_server."

Currently, a config.sxp file of a managed domain is not migrated to a
destination server even if the migration of the managed domain
succeeds.  The config.sxp file is kept in a source server.

By the patch, the config.sxp file is migrated with the managed domain.
The config.sxp file is unregistered from the source server, then the
config.sxp file is registered to the destination server.

BTW, should the config.sxp file be always migrated without the option?
If the managed domain is migrated without the option, the managed
domains exist on both the source server and the destination server.
(Of course, the managed domain on the source server is "halted" state,
and the managed domain on the destination server is "running" state.)
Is it good that the managed domains with a same UUID exist on both
servers?  (In the patch, I added the option for compatible.)

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>