qm: add section about machine types

qm: add section about machine types

expanding from the two currently existing sentences. In the first one, a
typo VMs -> VM's is fixed. In the second one, "one wants to" is changed
to "you want to", because the sentence already starts with "You can" and
it's active voice.

Adds information about the machine version, rationale behind the
defaults for it and what to do for deprecated versions.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>