vpci/msi: fix update of bound MSI interrupts

vpci/msi: fix update of bound MSI interrupts

Current update process of already bound MSI interrupts is wrong
because unmap_domain_pirq calls pci_disable_msi, which disables MSI
interrupts on the device. On the other hand map_domain_pirq doesn't
enable MSI, so the current update process of already enabled MSI
entries is wrong because MSI control bit will be disabled by
unmap_domain_pirq and not re-enabled by map_domain_pirq.

In order to fix this avoid unmapping the PIRQs and just update the
binding of the PIRQ. A new arch helper to do that is introduced.

Note that MSI-X is not affected because unmap_domain_pirq only
disables the MSI enable control bit for the MSI case, for MSI-X the
bit is left untouched by unmap_domain_pirq.

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