xen/blkback: Disable DISCARD support for loopback device (but leave for phy).

xen/blkback: Disable DISCARD support for loopback device (but leave for phy).

Until we back-port the changes from 3.3 which alter the loopback device
to support the full gamma of discard attributes. Otherwise we have to
punch through loop device to retrieve the underlaying disk size and
do other nasty things to get the proper information.

Also there is the outstanding issue that Logical Volumes won't pass
through the DISCARD support, so in most cases we can't take advantage of
this code until that gets fixed.

Fixes Oracle BZ#13779884
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>