KVM: x86: Refactor handling of SIPI_RECEIVED when setting MP_STATE

KVM: x86: Refactor handling of SIPI_RECEIVED when setting MP_STATE

Convert the incoming mp_state to INIT_RECIEVED instead of manually calling
kvm_set_mp_state() to make it more obvious that the SIPI_RECEIVED logic is
translating the incoming state to KVM's internal tracking, as opposed to
being some entirely unique flow.

Opportunistically add a comment to explain what the code is doing.

No functional change intended.

Link: https://lore.kernel.org/r/20250605195018.539901-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>