migration: add support for migrateURI configuration

migration: add support for migrateURI configuration

For now, we set the migration URI via command line '--migrate_uri' or
construct the URI by looking up the dest host's hostname which could be
solved by DNS automatically.

But in cases the dest host have two or more NICs to reach, we may need to
send the migration data over a specific NIC which is different from the
automatically resolved one for some reason like performance, security, etc.
Thus we must explicitly specify the migrateuri in command line everytime,
but it is too troublesome if there are many such hosts (and don't forget
virt-manager).

This patch adds a configuration file option on dest host to save the
default value set which can be specified to a migration hostname or
one of this host's addresses used for transferring data, thus user doesn't
have to specify it in command line everytime.

Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>