Since guest could re-program PCI device MSI-X table BAR, we should add mmio
emulation handler unregister.
However, after add unregister_mmio_emulation_handler API, emul_mmio_regions
is no longer accurate. Just replace it with max_emul_mmio_regions which records
the max index of the emul_mmio_node.
Tracked-On: #3475
Signed-off-by: Li, Fei1 <fei1.li@intel.com>