xen-mapcache: don't unmap locked entry during mapcache invalidation

When an IOREQ_TYPE_INVALIDATE is sent to QEMU, it invalidates all entry
of the map cache even if it's locked.

QEMU is not able to know that entry was invalidated, so when an IO
access is requested a segfault occured.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
