virsh: better support double quote

virsh: better support double quote

In origin code, double quote is only allowed at the begin or end
"complicated argument"
--some_opt="complicated string"  (we split this argument into 2 parts,
option and data, the data is "complicated string").

This patch makes it allow double quote at any position of
an argument:
complicated" argument"
complicated" "argument
--"some opt=complicated string"

This patch is also needed for the following patches,
the following patches will not split option argument into 2 parts,
so we have to allow double quote at any position of an argument.

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