ioemu: fix disk format security vulnerability

* make the xenstore reader in qemu-dm's startup determine which
  of qemu's block drivers to use according to the xenstore
  backend `type' field.  This `type' field typically comes from
  the front of the drive mapping string in ioemu.  The
  supported cases are:
    xm config file string      `type'  image format    qemu driver
     phy:[/dev/]<device>        phy     raw image       bdrv_raw
     file:<filename>            file    raw image       bdrv_raw
     tap:aio:<filename>         tap     raw image       bdrv_raw
     tap:qcow:<image>           tap     not raw         autoprobe
     tap:<cow-fmt>:<image>      tap     named format    bdrv_<cow-fmt>
  It is still necessary to autoprobe when the image is specified as
  `tap:qcow:<image>', because qemu distinguishes `qcow' and `qcow2'
  whereas blktap doesn't; `qcow' in xenstore typically means what
  qemu calls qcow2.  This is OK because qemu can safely distinguish
  the different cow formats provided we know it's not a raw image.

* Make the format autoprobing machinery never return `raw'.  This has
  two purposes: firstly, it arranges that the `tap:qcow:...' case
  above can be handled without accidentally falling back to raw
  format.  Secondly it prevents accidents in case the code changes in
  future: autoprobing will now always fail on supposed cow files which
  actually contain junk, rather than giving the guest access to the
  underlying file.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

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