qmp: fix handling of cmd with equal mark in qmp-shell
qmp-shell splits the argument and value of input command
by equal mark("="). But there are commands whose values
include equal mark themselves, and the json built by
qmp-shell will not correct. For example, when using NBD as
the target of block-backup command, the input
"block-backup target=nbd+unix:///drive0?socket=/tmp/nbd.sock"
will fail, because the json built will be as follows: