x86/hvm: convert gsi_assert_count into a variable size array

x86/hvm: convert gsi_assert_count into a variable size array

Rearrange the fields of hvm_irq so that gsi_assert_count can be converted into
a variable size array and add a new field to account the number of GSIs.

Due to this changes the irq member in the hvm_domain struct also needs to
become a pointer set at runtime.

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