libs/light: Propagate libxl__arch_domain_create() return code

libs/light: Propagate libxl__arch_domain_create() return code

Commit 34990446ca91 started to overwrite the `rc` value from
libxl__arch_domain_create(), thus error aren't propagated anymore.

Check `rc` value before doing the next thing.

Fixes: 34990446ca91 ("libxl: don't ignore the return value from xc_cpuid_apply_policy")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>