libxl: libxl_domain_restore: Put fd back to blocking mode

libxl: libxl_domain_restore: Put fd back to blocking mode

libxl_domain_restore calls, indirectly, xc_domain_restore.  The
latter, when doing a live migration, sets the fd from blocking mode
(which it must be on entry, or things go wrong) to nonblocking mode
and leaves it this way.  Arguably this is a bug in libxc, but to avoid
disrupting any callers we fix it in libxl.

So libxl_domain_restore now puts the fd back into blocking mode
before returning.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>