This patch adds support for kvm_gfn_to_hva_cache_init functions for
reads and writes that will cross a page.  If the range falls within
the same memslot, then this will be a fast operation.  If the range
is split between two memslots, then the slower kvm_read_guest and
kvm_write_guest are used.

Tested: Test against kvm_clock unit tests.
Signed-off-by: NAndrew Honig <ahonig@google.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>