nbd: Fix interpretation of the export flags

nbd: Fix interpretation of the export flags

The export flags are a 16 bit value, so be16_to_cpu() has to be used to
interpret them correctly. This makes discard and flush actually work
for named NBD exports (they did not work before, because the client
always assumed them to be unsupported because of the bug fixed by this
patch).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <1424887718-10800-20-git-send-email-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>