kvmtool assumes that the BAR that holds the address for the MSIX table
and PBA structure has a size which is equal to the total size of the
table and the PBA structure and it allocates memory from MMIO space
accordingly.  However, when initializing the BARs, the BAR size is set
according to the region size reported by VFIO. When the physical BAR
size is greater than what kvmtool allocated, we can have a situation
where the BAR overlaps with another BAR, in which case kvmtool will fail
to map the memory. This was found when trying to do PCI passthrough on a
PCIe Realtek r8168 NIC.  Let's fix this by allocating an amount of MMIO
memory equal to table + PBA size or BAR size, whichever is greater.