Support DriveMount API in CreateVM.

This implements the proposal found in docs/drive-mounts-proposal.md, with the
exception for supporting RateLimiters and IsReadOnly in the DriveMount objects,
due to the need for further refactoring of the internal stub drive code (will
be followed up in firecracker-microvm#296).

Signed-off-by: Erik Sipsma <[email protected]>

Author: sipsma