x86/vtd: Drop struct iommu_flush

x86/vtd: Drop struct iommu_flush

It is unclear why this abstraction exists, but iommu_get_flush() returns
possibly NULL and every user unconditionally dereferences the result.  In
practice, I can't spot a path where iommu is NULL, so I think it is mostly
dead.

Move the two function pointers into struct vtd_iommu (using a flush prefix),
and delete iommu_get_flush().  Furthermore, there is no need to pass the IOMMU
pointer to the callbacks via a void pointer, so change the parameter to be
correctly typed as struct vtd_iommu.  Clean up bool_t to bool in surrounding
context.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>