commandtest: Test virCommandSetSendBuffer() with virCommandDoAsyncIO()

Introduce a test case which ensures that a daemonized process can
work with virCommandSetSendBuffer() when async IO is enabled.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>

tests/commanddata/test29.log

@@ -0,0 +1,20 @@
+ARG:--close-stdin
+ARG:--check-daemonize
+ARG:--readfd
+ARG:3
+ENV:DISPLAY=:0.0
+ENV:HOME=/home/test
+ENV:HOSTNAME=test
+ENV:LANG=C
+ENV:LOGNAME=test
+ENV:PATH=/usr/bin:/bin
+ENV:TMPDIR=/tmp
+ENV:USER=test
+FD:0
+FD:1
+FD:2
+FD:3
+FD:6
+DAEMON:yes
+CWD:/
+UMASK:0022