pcib: Allocate the memory BAR with the MSI-X table.

This is required for pci_alloc_msix() to work and to thus use
MSI-X interrupts for PCI-e hotplug.

Reported by: cperciva
Reviewed by: cperciva
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40581

(cherry picked from commit e6b838363fb473b5e35a8ae6a1da5e15f5b52960)