x86/vioapic: allow the vIO APIC to have a variable number of pins

x86/vioapic: allow the vIO APIC to have a variable number of pins

Although it's still always set to VIOAPIC_NUM_PINS (48).

Add a new field to the hvm_ioapic struct to contain the number of pins (number
of IO redirection table entries) and turn the redirection table into a variable
sized array.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>