This patch adds a new qemu-io tool that links against the block layer and
image formats and allow to exercise them without needing a guest image.
It is inspired by the xfs_io tool which does the same for plain file I/O.
In fact the libxcmd library which is the backend of xfs_io is reused by this
tool in a limited fashing (cmd.[ch] files).
This version tests out most of the plain block I/O commands with the
most notable absent commands beeing snapshot handling and real aio.
This tool is the basis of the I/O path test suite I'm working on right now.
Signed-off-by: Christoph Hellwig <address@hidden>