block: Dont ignore previously set bdrv_flags

bdrv_flags is set by bdrv_parse_discard_flags(), but later it is reset
to zero.

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Message-id: 1376483201-13466-1-git-send-email-mohan@in.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Merge branch 'master' of git://git.qemu.org/qemu