Don't set SpiceSession::cmain twice during migration
Don't set SpiceSession::cmain twice during migration

During migration, migrate_connect() ends with:
spice_session_set_main_channel(mig->session,
                               migrate_channel_connect(mig, SPICE_CHANNEL_MAIN, 0));

migrate_channel_connect() calls spice_channel_new() which will create a
new SpiceMainChannel instance.
spice_channel_constructed() will be called during this instanciation,
which will call spice_session_channel_new(), which will set
SpiceSession::cmain for SpiceMainChannel instances.

When calling spice_session_set_main_channel(), SpiceSession::cmain will
thus already be set, which causes a runtime warning as
spice_session_set_main_channel() has a precondition checking that
SpiceSession::cmain is not set already.

This commit removes the call to spice_session_set_main_channel() from
migrate_connect() as this call is not doing anything more than the call
to migrate_channel_connect().

elmarco/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,elmarco/spice-gtk