Implement the FLUSH operation in the virtio-block emulation.

This gets rid of the following error message during FreeBSD guest bootup:
"vtbd0: hard error cmd=flush fsbn 0"

Reported by:	rodrigc
Reviewed by:	grehan


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272710 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f