PVE2 PHP Class: Use the ACPI (off/on/etc) commands; Expect PVE2_Exception; etc.

[lsthompson]

CpuID/pve2-api-php-client#42 for our evolving requested (draft) changes.

CpuID/pve2-api-php-client@b7ee78d

[lsthompson]

@CpuID has very kindly added me as a maintainer of his PHP Class repo ahead of 2024.

Will work with him and the other maintainer to improve the error reporting from the class, then finalise here.