x86/HVM: eliminate vulnerabilities from hvm_inject_msi()

x86/HVM: eliminate vulnerabilities from hvm_inject_msi()

- pirq_info() returns NULL for a non-allocated pIRQ, and hence we
  mustn't unconditionally de-reference it, and we need to invoke it
  another time after having called map_domain_emuirq_pirq()
- don't use printk(), namely without XENLOG_GUEST, for error reporting

This is XSA-96.

Signed-off-by: Jan Beulich <jbeulich@suse.com>