rsync: handle EAGAIN properly
rsync: handle EAGAIN properly

Go's io.Copy() is not able to handle EAGAIN but Read() and Write() are. We need
to keep retrying on EAGAIN.

Closes #3168.

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

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