KVM: x86: Let the guest own CR4.FSGSBASE

Add FSGSBASE to the set of possible guest-owned CR4 bits, i.e. let the
guest own it on VMX.  KVM never queries the guest's CR4.FSGSBASE value,
thus there is no reason to force VM-Exit on FSGSBASE being toggled.

Note, because FSGSBASE is conditionally available, this is dependent on
recent changes to intercept reserved CR4 bits and to update the CR4
guest/host mask in response to guest CPUID changes.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
[sean: added justification in changelog]
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200930041659.28181-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>