qemu-img convert makes a number of allocation requests, and less
common than the "Resource temporarily unavailable" error is
"Cannot allocate memory". The first seems to be largely raised
upon a thread creation for the image conversion, where the first
appears to be on buffer setup on the original process.
Regardless, we should check for both and retry conversion
automatically.
rhbz# 1892773
Change-Id: If435ed0efe034d4bb99f0281ebc73840c0f38fb3
9be6afc751