MFC r280037:

MFC r280037:
Rewrite virtio block device driver to work asynchronously and use the block
I/O interface.

Asynchronous operation, based on r280026 change, allows to not block virtual
CPU during I/O processing, that on slow/busy storage can take seconds.
Use of recently improved block I/O interface allows to process multiple
requests same time, that improves random I/O performance on wide storages.

Benchmarks of virtual disk, backed by ZVOL on RAID10 pool of 4 HDDs, show
~3.5 times random read performance improvements, while no degradation on
linear I/O.  Guest CPU usage during test dropped from 100% to almost zero.

git-svn-id: svn://svn.freebsd.org/base/stable/10@280744 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f