KVM: arm64: Disambiguate support for vSGIs v. vLPIs

vgic_supports_direct_msis() is a bit of a misnomer, as it returns true
if either vSGIs or vLPIs are supported. Pick it apart into a few
predicates and replace some open-coded checks for vSGIs, including an
opportunistic fix to always check if the CPUIF is capable of handling
vSGIs.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250724062805.2658919-2-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>