govc: Use unique searchFlagKey when calling NewSearchFlag

[makelarisjr]

Description

When cloning a VM if you specify the -host.dns flag it throws the error expected VirtualMachine entity, got HostSystem, this is because the SearchFlag instance is saved in the context with a static searchFlagKey

Closes: #2849

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Build related change

How Has This Been Tested?

I successfully cloned a VM by using this command:
./govc vm.clone -vm "TestDHCP" -host.dns "esxi-eu-dev-01.example.com" testvm

Checklist:

• My code follows the CONTRIBUTION guidelines of
  this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[vmwclabot]

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[dougm]

Thanks @makelarisjr ! Please go ahead and squash the commits and I will merge

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@makelarisjr, VMware has rejected your signed contributor license agreement. The change has already been merged, but it will be backed out by the project maintainers if the agreement is not resigned in a timely manner. Click here to resign the agreement.

[vmwclabot]

@makelarisjr, VMware has rejected your signed contributor license agreement. The change has already been merged, but it will be backed out by the project maintainers if the agreement is not resigned in a timely manner. Click here to resign the agreement.

[vmwclabot]

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