x86: avoid needless EPT table ajustment and cache flush

x86: avoid needless EPT table ajustment and cache flush

When a guest change it's MTRR MSRs, ajusting EPT table and flushing
cache are needed only when guest has IOMMU device, using need_iommu(d)
can minimize the impact to guest with device assigned, since a guest
may be hot plugged with a device thus there may be dirty cache lines
before need_iommu(d) becoming true, force the p2m_memory_type_changed
and flush_all when the first device is assigned to guest to amend this
issue.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Signed-off-by: Yang Zhang <yang.z.zhang@intel.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>