KVM: nVMX: Allow emulating RDPID on behalf of L2

KVM: nVMX: Allow emulating RDPID on behalf of L2

Return X86EMUL_CONTINUE instead X86EMUL_UNHANDLEABLE when emulating RDPID
on behalf of L2 and L1 _does_ expose RDPID/RDTSCP to L2.  When RDPID
emulation was added by commit fb6d4d340e05 ("KVM: x86: emulate RDPID"),
KVM incorrectly allowed emulation by default.  Commit 07721feee46b ("KVM:
nVMX: Don't emulate instructions in guest mode") fixed that flaw, but
missed that RDPID emulation was relying on the common return path to allow
emulation on behalf of L2.

Fixes: 07721feee46b ("KVM: nVMX: Don't emulate instructions in guest mode")
Link: https://lore.kernel.org/r/20250201015518.689704-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>