qcow2: Handle failure for potentially large allocations

qcow2: Handle failure for potentially large allocations

Some code in the block layer makes potentially huge allocations. Failure
is not completely unexpected there, so avoid aborting qemu and handle
out-of-memory situations gracefully.

This patch addresses the allocations in the qcow2 block driver.

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>