block: add VIRTIO_BLK_F_TOPOLOGY feature

Now virtio-block devices always advertise VIRTIO_BLK_F_TOPOLOGY feature.
Users can specify new `topology` option for the block device to set this
value. If the option is not set, Firecracker will check if the backing
file is actually a block device and try getting values from the host.
If it is not a block device or if obtaining information failed, use
default values instead.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>