x86/vPCI: tolerate (un)masking a disabled MSI-X entry

x86/vPCI: tolerate (un)masking a disabled MSI-X entry

None of the four reasons causing vpci_msix_arch_mask_entry() to get
called (there's just a single call site) are impossible or illegal prior
to an entry actually having got set up:
- the entry may remain masked (in this case, however, a prior masked ->
  unmasked transition would already not have worked),
- MSI-X may not be enabled,
- the global mask bit may be set,
- the entry may not otherwise have been updated.
Hence the function asserting that the entry was previously set up was
simply wrong. Since the caller tracks the masked state (and setting up
of an entry would only be effected when that software bit is clear),
it's okay to skip both masking and unmasking requests in this case.

Fixes: d6281be9d0145 ('vpci/msix: add MSI-X handlers')
Reported-by: Manuel Bouyer <bouyer@antioche.eu.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: Manuel Bouyer <bouyer@antioche.eu.org>