Skip clearing and restoring IA32_DEBUGCTL when the saved host value
is zero.
VM exit clears the register. If VM entry fails, the entry path has
already cleared any nonzero host value, so restoring zero is also
unnecessary in that case. Preserve the existing writes for nonzero
host state.
Avoiding these writes can also avoid intercepted MSR accesses when
bhyve runs under another hypervisor.
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