x86/domctl: have XEN_DOMCTL_getpageframeinfo3 preemptible

x86/domctl: have XEN_DOMCTL_getpageframeinfo3 preemptible

This hypercall can take a long time to finish because it attempts to
grab the `hostp2m' lock up to 1024 times. The accumulated wait for the
lock can take several seconds.

This can easily happen with a guest with 32 vcpus and plenty of RAM,
during localhost migration.

While the patch doesn't fix the problem with the lock contention and
the fact that the `hostp2m' lock is currently global (and not on a
single page), it is still an improvement to the hypercall. It will in
particular, down the road, allow dropping the arbitrary limit of 1024
entries per request.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>