refactor: create QemuServer::Monitor for high-level QMP access

refactor: create QemuServer::Monitor for high-level QMP access

QMP and monitor helpers are moved from QemuServer.pm.

By using only vm_running_locally instead of check_running, a cyclic
dependency to QemuConfig is avoided. This also means that the $nocheck
parameter serves no more purpose, and has thus been removed along with
vm_mon_cmd_nocheck.

Care has been taken to avoid errors resulting from this, and
occasionally a manual check for a VM's existance inserted on the
callsite.

Methods have been renamed to avoid redundant naming:
* vm_qmp_command -> qmp_cmd
* vm_mon_cmd -> mon_cmd
* vm_human_monitor_command -> hmp_cmd

mon_cmd is exported since it has many users. This patch also changes all
non-package users of vm_qmp_command to use the mon_cmd helper. Includes
mocking for tests.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>