memory: fix return value handing of guest_remove_page()

memory: fix return value handing of guest_remove_page()

Despite the description in mm.h, guest_remove_page() previously returned 0 for
paging errors.

Switch guest_remove_page() to having regular 0/-error semantics, and propagate
the return values from clear_mmio_p2m_entry() and mem_sharing_unshare_page()
to the callers (although decrease_reservation() is the only caller which
currently cares).

This is part of XSA-222.

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