x86/HVM: properly propagate errors from HVMOP_inject_msi

x86/HVM: properly propagate errors from HVMOP_inject_msi

There are a number of ways this operation can go wrong, all of which
got ignored so far.

In the context of this I wonder whether map_domain_emuirq_pirq()
returning 0 in the "already mapped" case is really intended to be that
way (this is why the subsequent NULL check here can't be an ASSERT()).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>