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
Approved by: so
Security: FreeBSD-EN-23:10.pci
Differential Revision: https://reviews.freebsd.org/D40581

(cherry picked from commit e6b838363fb473b5e35a8ae6a1da5e15f5b52960)
(cherry picked from commit 12ce57e6d3e770db9ca83a8cbaa8f332515ce3d7)