KVM: VMX: Treat UMIP as emulated if and only if the host doesn't have UMIP

Advertise UMIP as emulated if and only if the host doesn't natively
support UMIP, otherwise vmx_umip_emulated() is misleading when the host
_does_ support UMIP.  Of the four users of vmx_umip_emulated(), two
already check for native support, and the logic in vmx_set_cpu_caps() is
relevant if and only if UMIP isn't natively supported as UMIP is set in
KVM's caps by kvm_set_cpu_caps() when UMIP is present in hardware.

That leaves KVM's stuffing of X86_CR4_UMIP into the default cr4_fixed1
value enumerated for nested VMX.  In that case, checking for (lack of)
host support is actually a bug fix of sorts, as enumerating UMIP support
based solely on descriptor table exiting works only because KVM doesn't
sanity check MSR_IA32_VMX_CR4_FIXED1.  E.g. if a (very theoretical) host
supported UMIP in hardware but didn't allow UMIP+VMX, KVM would advertise
UMIP but not actually emulate UMIP.  Of course, KVM would explode long
before it could run a nested VM on said theoretical CPU, as KVM doesn't
modify host CR4 when enabling VMX, i.e. would load an "illegal" value into
vmcs.HOST_CR4.

Reported-by: Robert Hoo <robert.hu@intel.com>
Link: https://lore.kernel.org/all/20230310125718.1442088-2-robert.hu@intel.com
Link: https://lore.kernel.org/r/20230413231914.1482782-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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