KVM: x86: Bury guest_cpuid_is_amd_or_hygon() in cpuid.c

KVM: x86: Bury guest_cpuid_is_amd_or_hygon() in cpuid.c

Move guest_cpuid_is_amd_or_hygon() into cpuid.c now that, except for one
Intel quirk in the emulator, KVM checks for AMD vs. Intel *compatible*
vCPUs, not exact vendors, i.e. now that there should not be any reason for
KVM at-large to care about the exact vendor.

Opportunistically refactor the guts of the helper to use "entry" instead
of "best", and short circuit the !entry path to make the common case more
readable.

Link: https://lore.kernel.org/r/20240405235603.1173076-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>