passthrough: streamline _hvm_dirq_assist()

passthrough: streamline _hvm_dirq_assist()

The loop inside this function was calling two functions with loop-
invariable arguments which clearly don't need calling more than once:
send_guest_pirq() and __msi_pirq_eoi(). After moving these out of the
loop it further became apparent that folding the hvm_pci_msi_assert()
helper into the main function can further help readability.

In the course of this I noticed that __hvm_dpci_eoi() called
hvm_pci_intx_deassert() unconditionally, whereas hvm_pci_intx_assert()
(correctly) got called only when !hvm_domain_use_pirq(), so the former
is being made conditional now too.

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