Properly track the different reasons new I/O is temporarily disabled, and
only re-enable I/O when all reasons have cleared.

sys/dev/xen/blkfront/block.h:
	In the block front driver softc, replace the boolean
	XBDF_FROZEN flag with a count of commands and driver global
	issues that freeze the I/O queue.  So long xbd_qfrozen_cnt
	is non-zero, I/O is halted.

	Add flags to xbd_flags for tracking grant table entry and
	free command resource shortages.  Each of these classes can
	increment xbd_qfrozen_cnt at most once.

	Add a command flag (XBDCF_ASYNC_MAPPING) that is set whenever
	the initial mapping attempt of a command fails with EINPROGRESS.

sys/dev/xen/blkfront/blkfront.c:
	In xbd_queue_cb(), use new XBDCF_ASYNC_MAPPING flag to definitively
	know if an async bus dmamap load has occurred.

	Add xbd_freeze() and xbd_thaw() helper methods for managing
	xbd_qfrozen_cnt and use them to implement all queue freezing logic.

	Add missing "thaw" to restart I/O processing once grant references
	become available.

Sponsored by:	Spectra Logic Corporation