qemu-img: dynamically adjust iobuffer size during convert

qemu-img: dynamically adjust iobuffer size during convert

since the convert process is basically a sync operation it might
be benificial in some case to change the hardcoded I/O buffer
size to a greater value.

This patch increases the I/O buffer size if the output
driver advertises an optimal transfer length or discard alignment
that is greater than the default buffer size of 2M.

Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Peter Lieven <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>