tests/qtest/migration: Replace migrate_get_connect_uri inplace of migrate_get_socket_...

tests/qtest/migration: Replace migrate_get_connect_uri inplace of migrate_get_socket_address

Refactor migrate_get_socket_address to internally utilize 'socket-address'
parameter, reducing redundancy in the function definition.

migrate_get_socket_address implicitly converts SocketAddress into str.
Move migrate_get_socket_address inside migrate_get_connect_uri which
should return the uri string instead.

Signed-off-by: Het Gala <het.gala@nutanix.com>
Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240312202634.63349-4-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>