p2m/ept: add _subtree suffix to ept_invalidate_emt

p2m/ept: add _subtree suffix to ept_invalidate_emt

So that the name implies the function is used to walk the page table
pointer passed as parameter. Drop the parent_ prefix from the level
parameter, since the level passed is the one matching the EPT entry
passed in the mfn parameter.

While there also change bool_t to bool and add an assert to make sure
no level 0 entries (ie: 4K EPT leaf entries) are passed as parameters.

No functional change intended.

Suggested-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>