Fix memShared units when reported from MOM

MOM reports the ksm_pages_sharing value in raw pages.
Both VDSM and engine expect megabytes, so this patch adds
the necessary conversion.

This also moves the PAGE_SIZE_BYTES to caps.py to avoid
circular dependency.

Change-Id: Ie6a68221d74192c38de158fca4d5f96f268150ab
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1072030
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28115
Reviewed-by: Dan Kenigsberg <danken@redhat.com>