x86/mm: Improvements to PV l1e mapping helpers

x86/mm: Improvements to PV l1e mapping helpers

Drop guest_unmap_l1e() and use unmap_domain_page() directly.  This will
simplify future cleanup.  Rename guest_map_l1e() to map_guest_l1e() to closer
match the mapping nomenclature.

Switch map_guest_l1e() to using mfn_t.  Correct the comment to indicate that
it takes a linear address (not a virtual address), and correct the parameter
name.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>