KVM: x86: Pull CPUID capabilities from boot_cpu_data only as needed

Don't memcpy() all of boot_cpu_data.x86_capability, and instead explicitly
fill each kvm_cpu_cap_init leaf during kvm_cpu_cap_init().  While clever,
copying all kernel capabilities risks over-reporting KVM capabilities,
e.g. if KVM added support in __do_cpuid_func(), but neglected to init the
supported set of capabilities.

Note, explicitly grabbing leafs deliberately keeps Linux-defined leafs as
0!  KVM should never advertise Linux-defined leafs; any relevant features
that are "real", but scattered, must be gathered in their correct hardware-
defined leaf.

Link: https://lore.kernel.org/r/20241128013424.4096668-54-seanjc@google.com


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