cli: add missing remove_first method for --blkiotune

Add missing remove_first method for --blkiotune.
Update related man pages.

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>

man/virt-install.pod

@@ -164,7 +164,7 @@ Use --memtune=? to see a list of all available sub options. Complete details at
 
 Tune blkio policy for the domain process. Example invocations
 
-    --blkiotune weight=100
+    --blkiotune 100
     --blkiotune weight=100,device_path=/dev/sdc,device_weight=200
 
 Use --blkiotune=? to see a list of all available sub options. Complete details at L<http://libvirt.org/formatdomain.html#elementsBlockTuning>

virtinst/cli.py

@@ -2026,6 +2026,7 @@ class ParserPanic(VirtCLIParser):
 class ParserBlkiotune(VirtCLIParser):
     def _init_params(self):
         self.clear_attr = "blkiotune"
+        self.remove_first = "weight"
 
         self.set_param("blkiotune.weight", "weight")
         self.set_param("blkiotune.device_path", "device_path")