create_disks: refactor out and improve efidisk creation

create_disks: refactor out and improve efidisk creation

factor out code in a new create_efidisk submethod, as else this code
is hardly readable as the efidisk0 case is a special case. Refer from
putting all this specialised handling directly to the much shorter
code for all other cases.

Also the disk was created with a specific format and then a format
detection on the newly created disk was done, which is pretty
useless, clear that up.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>