libxl: handle errors from xc_sharing_* info functions

libxl: handle errors from xc_sharing_* info functions

On a 32 bit hypervisor xl info currently reports:
sharing_freed_memory   : 72057594037927935
sharing_used_memory    : 72057594037927935

Eat the ENOSYS and turn it into 0. Log and propagate other errors.

I don't have a 32 bit system handy, so tested on x86_64 with a libxc
hacked to return -ENOSYS and -EINVAL.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>