tools/libxc: retry hypercall in case of EFAULT

tools/libxc: retry hypercall in case of EFAULT

A hypercall issued via the privcmd driver can very rarely return
-EFAULT even if the hypercall buffers are locked in memory. This
happens for hypercall buffers in user memory when the Linux kernel
is doing memory scans e.g. for page migration or compaction.

Retry the getpageframeinfo3 hypercall up to 2 times in case
-EFAULT is returned and the hypervisor might see invalid PTEs for
user hypercall buffers (which should be the case only if the kernel
doesn't offer a /dev/xen/hypercall node).

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>