Add mechanism to select per block device cache strategy.

[sandreim]

We should investigate the opportunity and possibilities to allow customers to influence how host storage is being accessed in the Firecracker block device emulation. Here is a list of switches that might make sense to expose as configuration options:

• ignore flush commands (from guest virtio driver) for performance reasons
• use host page cache for read/write operations or prefer direct io with the block file
• synchronous mode (O_SYNC/O_DSYNC) - writes will be reported complete only when committed to the host storage device

[berciuliviu]

Relates to 2223