block: Save errno before error_setg_errno

error_setg_errno() may overwrite errno; therefore, its value should be
read before calling that function and not afterwards.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
block/vpc: fix virtual size for images created with disk2vhd

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>