cfg2cmd: fix +pveN machine types with pxe

cfg2cmd: fix +pveN machine types with pxe

Pinned machine versions like "pc-i440fx-4.2+pve2.pxe" would otherwise
get a second "+pve0" suffix, which is incorrect.

Also deal with non-pve pinned versions correctly, i.e.
"pc-i440fx-5.2.pxe" becomes "pc-i440fx-5.2+pve0.pxe".

Handle .pxe suffixes in Machine.pm as well, and add two test cases.

Co-developed-by: Luca Berneking <luca@berneking.net>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>