Conditionally set disk UUID if empty in backing info

[palnabarun]

Description

Previously, the UUID was always set to a new value even if it was already present. This patch ensures that the disk UUID is now set only if it is
currently empty in the backing information.

How Has This Been Tested?

Made changes to govmomi pulled by vmware-tanzu/vm-operator#1258 and tested whether the UUID if set, remains same. With this patch, the expected behavior is validated.

Adds an unit test to validate the contract.

❯ go test -v -timeout 30s -run ^TestVirtualDiskUUIDAssignment$ github.com/vmware/govmomi/simulator
=== RUN   TestVirtualDiskUUIDAssignment
=== RUN   TestVirtualDiskUUIDAssignment/empty_UUID
=== RUN   TestVirtualDiskUUIDAssignment/generated_UUID
=== RUN   TestVirtualDiskUUIDAssignment/explicit_UUID
--- PASS: TestVirtualDiskUUIDAssignment (0.44s)
    --- PASS: TestVirtualDiskUUIDAssignment/empty_UUID (0.16s)
    --- PASS: TestVirtualDiskUUIDAssignment/generated_UUID (0.05s)
    --- PASS: TestVirtualDiskUUIDAssignment/explicit_UUID (0.05s)
PASS
ok      github.com/vmware/govmomi/simulator     1.304s

Guidelines

Please read and follow the CONTRIBUTION guidelines of this project.

[palnabarun]

Failing test

=== RUN   ExampleHandlerREST
--- FAIL: ExampleHandlerREST (2.22s)
got:
session valid=true
session valid=true
session valid=true
session valid=true
session valid=false
session valid=false
want:
session valid=true
session valid=false
session valid=true
session valid=true
session valid=false
session valid=false
FAIL
FAIL	github.com/vmware/govmomi/session/keepalive	6.102s

[palnabarun]

Most likely a flake. Locally, the test passes.

~/s/v/govmomi on 🌱 vcsim/check-disk-backing-if-uuid-exists (1f70772) [📦] [💣 1]
❯ go test -v github.com/vmware/govmomi/session/keepalive
=== RUN   TestHandlerSOAP
--- PASS: TestHandlerSOAP (0.49s)
=== RUN   TestHandlerREST
--- PASS: TestHandlerREST (0.49s)
=== RUN   ExampleHandlerSOAP
--- PASS: ExampleHandlerSOAP (1.90s)
=== RUN   ExampleHandlerREST
--- PASS: ExampleHandlerREST (1.92s)
PASS
ok      github.com/vmware/govmomi/session/keepalive     5.234s

[palnabarun]

This is failing locally as well. Digging into it. Fixed

not ok 313 vm.clone
# (from function `assert_equal' in file ./test_helper.bash, line 232,
#  in test file ./vm.bats, line 939)
#   `assert_equal "$nitems" "$uitems"' failed
# expected: 9
# actual:   5