target-i386: fix Coverity complaints about overflows

sipi_vector is an int; it is shifted by 12 and passed as a 64-bit value,
which makes Coverity think that we wanted (uint64_t)sipi_vector << 12.

But actually it must be between 0 and 255.  Make this explicit.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
