x86/HVM: make vmsi_deliver() return proper error values

x86/HVM: make vmsi_deliver() return proper error values

... and propagate this from hvm_inject_msi(). In the course of this I
spotted further room for cleanup:
- vmsi_inj_irq()'s struct domain * parameter was unused
- vmsi_deliver() pointlessly passed on dest_ExtINT to vmsi_inj_irq()
  (which that one validly refused to handle)
- vmsi_inj_irq()'s sole caller guarantees a proper delivery mode (i.e.
  rather than printing an obscure message we can just BUG())
- some formatting and log message quirks

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