virsh: rework command parsing

virsh: rework command parsing

Old virsh command parsing mashes all the args back into a string and
miss the quotes, this patches fix it. It is also needed for introducing
qemu-monitor-command which is very useful.

This patches uses the new vshCommandParser abstraction and adds
vshCommandArgvParse() for arguments vector, so we don't need
to mash arguments vector into a command sting.

And the usage was changed:
old:
virsh [options] [commands]

new:
virsh [options]... [<command_string>]
virsh [options]... <command> [args...]

So we still support commands like:
"define D.xml; dumpxml D" was parsed as a commands-string.

and support commands like:
we will not mash them into a string, we use new argv parser for it.

But we don't support the command like:
"define D.xml; dumpxml" was parsed as a command-name, but we have no such command-name.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>