In the Xen block front driver, take advantage of backends that

support cache flush and write barrier commands.

sys/dev/xen/blkfront/block.h:
	Add per-command flag that specifies that the I/O queue must
	be frozen after this command is dispatched.  This is used
	to implement "single-stepping".

	Remove the unused per-command flag that indicates a polled
	command.

	Add block device instance flags to record backend features.

	Add a block device instance flag to indicate the I/O queue
	is frozen until all outstanding I/O completes.

	Enhance the queue API to allow the number of elements in a
	queue to be interrogated.

	Prefer "inline" to "__inline".

sys/dev/xen/blkfront/blkfront.c:
	Formalize queue freeze semantics by adding methods for both
	global and command-associated queue freezing.

	Provide mechanism to freeze the I/O queue until all outstanding
	I/O completes.  Use this to implement barrier semantics
	(BIO_ORDERED) when the backend does not support
	BLKIF_OP_WRITE_BARRIER commands.

	Implement BIO_FLUSH as either a BLKIF_OP_FLUSH_DISKCACHE
	command or a 0 byte write barrier.  Currently, all publicly
	available backends perform a diskcache flush when processing
	barrier commands, and this frontend behavior matches what
	is done in Linux.

	Simplify code by using new queue length API.

	Report backend features during device attach and via sysctl.

Submitted by:	Roger Pau Monné
Submitted by:	gibbs (Merge with new driver queue API, sysctl support)

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