Don't reset bs->is_temporary in bdrv_open_common

To fix https://bugs.launchpad.net/qemu/+bug/597402 where qemu fails to
call unlink() on temporary snapshots due to bs->is_temporary getting clobbered
in bdrv_open_common() after being set in bdrv_open() which calls the former.

We don't need to initialize bs->is_temporary in bdrv_open_common().

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
