target/i386/mshv: migrate Synic SINT MSRs

Migrate HyperV SynIC SINT MSRs. We can only read/write those if SCONTROL
is enabled in the guest, hence we have to split the SINT MSR out and
make reading/writing them dependent on that MSR.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Reviewed-by: Doru Blânzeanu <dblanzeanu@linux.microsoft.com>
Link: https://lore.kernel.org/r/20260710101534.664604-7-magnuskulke@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>