Collect io limits for ide, sata, scsi, virtio

[grin31]

add IoLimitCollector class scrape limits settings for qemu VM
labels:
['id', 'node', 'type', 'device', 'device_name']
example:

# HELP pve_mbps_wr Proxmox vm config mbps_wr
# TYPE pve_mbps_wr gauge
pve_mbps_wr{device="virtio",device_name="virtio0",id="qemu/100",node="pve01",type="qemu"} 20.0
# HELP pve_mbps_wr_max Proxmox vm config mbps_wr_max
# TYPE pve_mbps_wr_max gauge
pve_mbps_wr_max{device="virtio",device_name="virtio0",id="qemu/100",node="pve01",type="qemu"} 25.0
# HELP pve_iops_rd Proxmox vm config iops_rd
# TYPE pve_iops_rd gauge
pve_iops_rd{device="virtio",device_name="virtio1",id="qemu/100",node="pve01",type="qemu"} 1234.0
# HELP pve_mbps_rd Proxmox vm config mbps_rd
# TYPE pve_mbps_rd gauge
pve_mbps_rd{device="virtio",device_name="virtio0",id="qemu/102",node="pve01",type="qemu"} 13.0
# HELP pve_mbps_wr Proxmox vm config mbps_wr
# TYPE pve_mbps_wr gauge
pve_mbps_wr{device="virtio",device_name="virtio0",id="qemu/102",node="pve01",type="qemu"} 17.0

[grin31]

resolves #115

[znerol]

Closed, see #156