x86/MSI-X: cleanup

x86/MSI-X: cleanup

- __pci_enable_msix() now checks that an MSI-X capability was actually
  found
- pass "pos" to msix_capability_init() as both callers already know it
  (and hence there's no need to re-obtain it)
- call __pci_disable_msi{,x}() directly instead of via
  pci_disable_msi() from __pci_enable_msi{x,}() state validation paths
- use msix_control_reg() instead of open coding it
- log message adjustments
- coding style corrections

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>