x86/PCI: read MSI-X table entry count early

x86/PCI: read MSI-X table entry count early

Rather than doing this every time we set up interrupts for a device
anew (and then in two distinct places) fill this invariant field
right after allocating struct arch_msix.

While at it also obtain the MSI-X capability structure position just
once, in msix_capability_init(), rather than in each caller.

Furthermore take the opportunity and eliminate the multi_msix_capable()
alias of msix_table_size().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>