hyperv: SControl is optional to enable SynIc

hyperv: SControl is optional to enable SynIc

SynIc can be enabled regardless of the SControl mechanisim which can
register a GSI for a given SintRoute.

This behaviour can achived by setting enabling SIMP and then the guest
will poll on the message slot.

Once there is another message pending the host will set the message slot
with the pending flag.
When the guest polls from the message slot, in case the pending flag is
set it will write to the HV_X64_MSR_EOM indicating it has cleared the
slot and we can try and push our message again.

Signed-off-by: Jon Doron <arilou@gmail.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220216102500.692781-2-arilou@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>