raw: Probe required direct I/O alignment

raw: Probe required direct I/O alignment

Add a bs->request_alignment field that contains the required
offset/length alignment for I/O requests and fill it in the raw block
drivers. Use ioctls if possible, else see what alignment it takes for
O_DIRECT to succeed.

While at it, also expose the memory alignment requirements, which may be
(and in practice are) different from the disk alignment requirements.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>