qm: add remote-migrate command

qm: add remote-migrate command

which wraps the remote_migrate_vm API endpoint, but does the
precondition checks that can be done up front itself.

this now just leaves the FP retrieval and target node name lookup to the
sync part of the API endpoint, which should be do-able in <30s ..

an example invocation:

$ qm remote-migrate 1234 4321 'host=123.123.123.123,apitoken=PVEAPIToken=user@pve!incoming=aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee,fingerprint=aa:bb:cc:dd:ee:ff:aa:bb:cc:dd:ee:ff:aa:bb:cc:dd:ee:ff:aa:bb:cc:dd:ee:ff:aa:bb:cc:dd:ee:ff:aa:bb' --target-bridge vmbr0 --target-storage zfs-a:rbd-b,nfs-c:dir-d,zfs-e --online

will migrate the local VM 1234 to the host 123.123.1232.123 using the
given API token, mapping the VMID to 4321 on the target cluster, all its
virtual NICs to the target vm bridge 'vmbr0', any volumes on storage
zfs-a to storage rbd-b, any on storage nfs-c to storage dir-d, and any
other volumes to storage zfs-e. the source VM will be stopped but remain
on the source node/cluster after the migration has finished.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>