block: Polish error handling of brdv_open2 (Jan Kiszka)

Make sure that we always delete temporary disk images on error, remove
obsolete malloc error checks and return proper error codes.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6702 c046a42c-6fe2-441c-8c8c-71466251a162
