pvesh: proxy handler: fix handling array parameters

pvesh: proxy handler: fix handling array parameters

As reported in the community forum and reproduced locally, issuing a
QEMU guest agent command would lead to an error when proxying to
another node:

> root@pve8a2 ~ # pvesh create /nodes/pve8a1/qemu/126/agent/exec --command 'whoami'
> Wide character in die at /usr/share/perl5/PVE/RESTHandler.pm line 918.
> proxy handler failed: Agent error: Guest agent command failed, error was 'Failed to execute child process “ARRAY(0x55842bb161a0)” (No such file or directory)'

Fix it, by splitting up array references correctly.

[0]: https://forum.proxmox.com/threads/136520/

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>