tmt: use different disk size syntax

For some reason the previous syntax to specify the size for the
virtual machine provider [0] stopped working. This commit now
uses the syntax described in [1] instead which seems to work.

[0] https://tmt.readthedocs.io/en/stable/spec/plans.html#virtual
[1] https://tmt.readthedocs.io/en/stable/spec/hardware.html#disk

plans/all.fmf

@@ -2,13 +2,11 @@ summary: Run all tests inside a VM environment
 provision:
   how: virtual
   image: fedora:40
-  # 120 means 120GB, disk has to be specificed here, the format from:
-  #   https://tmt.readthedocs.io/en/stable/spec/hardware.html#disk
-  # does not work here or below under "hardware"
-  disk: 120
   hardware:
     virtualization:
       is-supported: true
+    disk:
+      - size: '>= 120 GB'
 prepare:
   how: install
   package: