CFT - Xen infrastructure and block I/O improvements

At Spectra Logic, we are using FreeBSD amd64 under Xen to serve storage
to other Xen domains.  Over the past 9 months, we've made several changes
to FreeBSD's Xen support.  These include:

 o Support for backend devices (e.g. blkback)
 o Extensions to the Xen para-virtualized block API to allow for larger
   and more outstanding I/Os.
 o A completely rewritten block back driver with support for fronting
   I/O to both raw devices and files.
 o General cleanup and documentation of the XenBus and XenStore support code.
 o Robustness and performance updates for the block front driver.
 o Fixes to the netfront driver.

Some of these changes have already been pushed back into FreeBSD, but the
bulk of them need additional testing, especially under i386 PV, before
they can be committed.  If you work in the Xen area, I'd appreciate your
review and/or testing of these changes.

http://people.freebsd.org/~gibbs/FreeBSD-head-xen-diffs_2010_09_16.txt

Thanks,
Justin