tests: Check QVIRTIO_F_ANY_LAYOUT flag in virtio-blk test

Check the QVIRTIO_F_ANY_LAYOUT flag before performing operations with 2
descriptor layout. This is to follow the specification strictly.

This patch depends on:
[PATCH v5 0/5] libqos: Virtio MMIO driver

Signed-off-by: Marc Marí <marc.mari.barcelo@gmail.com>
Message-id: 1424815154-27243-1-git-send-email-marc.mari.barcelo@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
