shr_pages field is MEM_SHARING-only

shr_pages field is MEM_SHARING-only

Conditionalize it and its uses accordingly. The main goal though is to
demonstrate that x86's p2m_teardown() is now empty when !HVM, which in
particular means the last remaining use of p2m_lock() in this cases goes
away.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tamas K Lengyel <tamas@tklengyel.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>