KVM: x86: avoid unnecessary rmap walks when creating/moving slots

On large memory guests it has been observed that creating a memslot
for a very large range can take noticeable amount of time.
Investigation showed that the time is spent walking the rmaps to update
existing sptes to remove write access or set/clear dirty bits to support
dirty logging.  These rmap walks are unnecessary when creating or moving
a memslot.  A newly created memslot will not have any existing mappings,
and the existing mappings of a moved memslot will have been invalidated
and flushed.  Any mappings established once the new/moved memslot becomes
visible will be set using the properties of the new slot.

Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Message-Id: <1591128450-11977-3-git-send-email-anthony.yznaga@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>