virsh: better handling the boolean option

virsh: better handling the boolean option

in old code the following commands are equivalent:
     virsh # dumpxml --update-cpu=vm1
     virsh # dumpxml --update-cpu vm1
because the old code split the option argument into 2 parts:
--update-cpu=vm1 is split into update-cpu and vm1,
and update-cpu is a boolean option, so the parser takes vm1 as another
argument, very strange.

after this patch applied, the first one will become illegal.

To achieve this, we don't parse/check options when parsing command sting,
but check options when parsing a command argument. And the argument is
not split when parsing command sting.

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