Lots of updates

• fix release_version of get_SRs_required_for_recover
• support SM backend-kind override via VDI.other_config key
• support floppy disk drives
• when performing a snapshot, set is_a_snapshot
• update API version to 2.4
• don't double-count HA-protected VMs when restarting them
• don't create refs for snapshots that don't exist on import
• expose RDP state, via guest agents, via VM guest metrics
• read all paths from the config file
• use 'ip' rather than 'ifconfig'
• block the pool-join if the joining host has a network bond
• use local force_state_reset after cross-pool SMX
• correct error text for assert_no_shared_srs_on_me
• allow ISO VBDs to be made from any SR
• disallow resume while a checkpoint is ongoing
• don't pause tapdisk
• use oclock rather than Unix.time
• import_raw_vdi: pass content-length to vhd-tool
• update hostname when it changes
• allow WLB servers to have an IPv6 address
• make sure we return the task reference when cancelling a task
• python: fix compatibility with python 2.7
• add a plugin to install supplemental packs
• create a PGPU for the system display device
• allow PCI vendors to be whitelisted for GPU passthrough
• allow passthrough of vga_arbiter device when appropriate
• remove VM hooks which are now handled by xenopsd
• set the content-length and close the tar header when exporting metadata
• handle cancellation of cross-host VDI.copy
• prevent VM.revert deleting all VDIs unconditionally
• update VDI.snapshot_of links after a VM.revert
• support for read-caching
• check for enough disk space before installing hotfixes
• VM.hard_reboot cancels VM.clean_reboot