x86/IRQ: make max number of guests for a shared IRQ configurable

x86/IRQ: make max number of guests for a shared IRQ configurable

... and increase the default to 16.

Current limit of 7 is too restrictive for modern systems where one GSI
could be shared by potentially many PCI INTx sources where each of them
corresponds to a device passed through to its own guest. Some systems do not
apply due dilligence in swizzling INTx links in case e.g. INTA is declared as
interrupt pin for the majority of PCI devices behind a single router,
resulting in overuse of a GSI.

Introduce a new command line option to configure that limit and dynamically
allocate an array of the necessary size. Set the default size now to 16 which
is higher than 7 but could later be increased even more if necessary.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>