x86: make get_page_from_l1e() return a proper error code

x86: make get_page_from_l1e() return a proper error code

... so that the guest can actually know the reason for the (hypercall)
failure.

ptwr_do_page_fault() could propagate the error indicator received from
get_page_from_l1e() back to the guest in the high half of the error
code (entry_vector), provided we're sure all existing guests can deal
with that (or indicate so by means of a to-be-added guest feature
flag). Alternatively, a second virtual status register (like CR2)
could be introduced.

Signed-off-by: Jan Beulich <jbeulich@novell.com>