xl: rename "list-vm" command to "vm-list"

xl: rename "list-vm" command to "vm-list"

All of the other "list" verbs are of the form "$noun-list". For
example: "pci-list", "vcpu-list", "network-list", "block-list", etc.

Additionally, many people have well trained muscle memory from years
of typing "xm li". "xl li" was ambiguous due to "xl list-vm", thus
resulting in "command not implemented".

Finally, this command was missing from the xl man page.

Signed-off-by: Matt Wilson <msw@amazon.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>