x86/HVM: serialize pinned cache attribute list manipulation

x86/HVM: serialize pinned cache attribute list manipulation

While the RCU variants of list insertion and removal allow lockless list
traversal (with RCU just read-locked), insertions and removals still
need serializing amongst themselves. To keep things simple, use the
domain lock for this purpose.

This is CVE-2022-42334 / part of XSA-428.

Fixes: 642123c5123f ("x86/hvm: provide XEN_DMOP_pin_memory_cacheattr")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 829ec245cf66560e3b50d140ccb3168e7fb7c945)