Skip clearing and restoring DEBUGCTL when the saved host value is
zero. The entry path leaves DEBUGCTL zero, and LBR virtualization
restores that host value on VM exit. A rejected guest entry also
returns with the pre-VMRUN DEBUGCTL value. Guest accesses to DEBUGCTL
remain intercepted.
Preserve the existing writes for nonzero host state. Avoiding the
redundant writes can also avoid intercepted MSR accesses when bhyve
runs under another hypervisor.
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