give error instead of disabling efidisk button

give error instead of disabling efidisk button

when seabios is selected, we do not want to simply
disable the add efi button, because it is not clear
why it is disabled

so we give a meaningful error message instead

the button is still 'just' disabled when there
is already an efidisk, because then it is clear
that you alread have one

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>