libxl: use init and dispose functions with libxl_physinfo

libxl: use init and dispose functions with libxl_physinfo

The typical pattern when calling libxl functions that populate a
structure is

  libxl_foo foo;
  libxl_foo_init(&foo);
  libxl_get_foo(ctx, &foo);
  ...
  libxl_foo_dispose(&foo);

Fix several instances of libxl_physinfo missing the init and
dispose calls.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>