lxd/migrate: MigrationSinkArgs push mode
lxd/migrate: MigrationSinkArgs push mode

Extend MigrationSinkArgs to support push mode.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@canonical.com>

erikmack/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,georghopp/lxd,hallyn/lxd,stgraber/lxd,chrisglass/lxd,chrisglass/lxd,stgraber/lxd,pcdummy/lxd,ollie314/lxd,stgraber/lxd,mjeanson/lxd,ollie314/lxd,stgraber/lxd,akshaykarle/lxd,techtonik/lxd,ollie314/lxd,hallyn/lxd,techtonik/lxd,hallyn/lxd,Skarlso/lxd,akshaykarle/lxd,jsavikko/lxd,hallyn/lxd,lxc/lxd,Skarlso/lxd,hallyn/lxd,Skarlso/lxd,erikmack/lxd,jsavikko/lxd,stgraber/lxd,lxc/lxd,mjeanson/lxd,pcdummy/lxd,lxc/lxd,akshaykarle/lxd,atxwebs/lxd,hallyn/lxd,mjeanson/lxd,chrisglass/lxd,atxwebs/lxd,atxwebs/lxd,georghopp/lxd,jsavikko/lxd,Skarlso/lxd,georghopp/lxd,lxc/lxd,lxc/lxd,erikmack/lxd,pcdummy/lxd,techtonik/lxd