configs,tests: Update refs to deprecated 'x86-ubuntu-img'

This resource has been deprecated in favor of 'x86-ubuntu-18.04-img'. In
practise, these are the same resource (i.e., the same disk image), but
we deprecate 'x86-ubuntu-img' as 'x86-ubuntu-18.04-img' is more explicit
on what the disk image contains. Going forward we may want different
disk image resources with different Ubuntu versions, and we want it to
be clear what the user is retrieving.

Change-Id: I06b08e6b65cf431067ffda35ebd10d419068d073
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54583
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/configs/boot_kvm_fork_run.py

@@ -203,7 +203,7 @@ motherboard.set_kernel_disk_workload(
         resource_directory=args.resource_directory,
     ),
     disk_image=Resource(
-        "x86-ubuntu-img",
+        "x86-ubuntu-18.04-img",
         resource_directory=args.resource_directory,
     ),
     readfile_contents=dedent(

tests/gem5/configs/boot_kvm_switch_exit.py

@@ -188,7 +188,7 @@ motherboard.set_kernel_disk_workload(
         resource_directory=args.resource_directory,
     ),
     disk_image=Resource(
-        "x86-ubuntu-img",
+        "x86-ubuntu-18.04-img",
         resource_directory=args.resource_directory,
     ),
     # The first exit signals to switch processors.

tests/gem5/configs/x86_boot_exit_run.py

@@ -207,7 +207,7 @@ motherboard.set_kernel_disk_workload(
         resource_directory=args.resource_directory,
     ),
     disk_image=Resource(
-        "x86-ubuntu-img",
+        "x86-ubuntu-18.04-img",
         resource_directory=args.resource_directory,
     ),
     kernel_args=kernal_args,