We can always assume raw, file and qcow2 being available; so do not use
bdrv_find_format() to locate their BlockDriver objects but statically
reference the respective objects.

Cc: qemu-stable@nongnu.org
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit ef810437)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>