KVM: x86: Open code vendor_intel() in string_registers_quirk()

Open code the is_guest_vendor_intel() check in string_registers_quirk() to
discourage makiking exact vendor==Intel checks in the emulator, and to
remove the rather awful #ifdeffery.

The string quirk is literally the only Intel specific, *non-architectural*
behavior that KVM emulates.  All Intel specific behavior that is
architecturally defined applies to all vendors that are compatible with
Intel's architecture, i.e. should use guest_cpuid_is_intel_compatible().

Link: https://lore.kernel.org/r/20240405235603.1173076-10-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>