x86/hyperv: Fix hv_get/set_register for nested bringup

hv_get_nested_reg only translates SINT0, resulting in the wrong sint
being registered by nested vmbus.

Fix the issue with new utility function hv_is_sint_reg.

While at it, improve clarity of hv_set_non_nested_register and hv_is_synic_reg.

Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Reviewed-by: Jinank Jain <jinankjain@linux.microsoft.com>
Link: https://lore.kernel.org/r/1675980172-6851-1-git-send-email-nunodasneves@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>