ANDROID: pkvm: x86: Hotfix non-working SMP in old guest kernels

Somehow I missed the fact that if a pVM hasn't issued the
PKVM_GHC_START_CPU hypercall, pKVM's sipi_vector remains uninitialized
(i.e. zero). So in order to prevent regressing pVM kernels that haven't
been updated to start CPUs via the hypercall to pKVM instead of
INIT/SIPI, it is not enough to temporarily allow the host to start a
secondary vCPU at any time, we also need to temporarily avoid overriding
the host's sipi_vector with pKVM's one (unless pKVM's sipi_vector is
already non-zero, so we are sure that the pVM has actually issued the
hypercall). Otherwise the vCPU starts executing random code at zero
address and crashes.

Bug: 374338925
Test: run microdroid or any other pVM with SMP whose kernel was compiled
without the patch aosp/3796083
Upstream-Task: 402758258

Fixes: f944ef34 ("ANDROID: pkvm: x86: Implement secure startup of secondary vCPUs")
Change-Id: I6338c0fd801691efe76e0f5d899fbcf9d26ecbf0
Signed-off-by: Dmytro Maluka <dmaluka@google.com>