xen/pci: Refactor MSI code that implements MSI functionality within XEN

xen/pci: Refactor MSI code that implements MSI functionality within XEN

On Arm, the initial plan is to only support GICv3 ITS which doesn't
require us to manage the MSIs because the HW will protect against
spoofing. Move the code under CONFIG_HAS_PCI_MSI flag to gate the code
for ARM.

No functional change intended.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>