x86/passthrough: introduce a flag for GSIs not requiring an EOI or unmask

x86/passthrough: introduce a flag for GSIs not requiring an EOI or unmask

There's no need to setup a timer for GSIs that are edge triggered,
since those don't require any EIO or unmask, and hence couldn't block
other interrupts.

Note this is only used by PVH dom0, that can setup the passthrough of
edge triggered interrupts from the vIO-APIC. One example of such kind
of interrupt that can be used by a PVH dom0 would be the RTC timer.

While there introduce an out label to do the unlock and reduce code
duplication.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>