tools/foreignmem: Support querying the size of a resource

tools/foreignmem: Support querying the size of a resource

With the Xen side of this interface (soon to be) fixed to return real sizes,
userspace needs to be able to make the query.

Introduce xenforeignmemory_resource_size() for the purpose, bumping the
library minor version.

Update both all osdep_xenforeignmemory_map_resource() implementations to
understand size requests, skip the mmap() operation, and copy back the
nr_frames field.

For NetBSD, also fix up the ioctl() error path to issue an unmap(), which was
overlooked by c/s 4a64e2bb39 "libs/foreignmemory: Implement on NetBSD".

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Wei Liu <wl@xen.org>