libvfio-user: bump MAX_DMA_REGIONS

Update libvfio-user for a higher number of supported regions, as needed
by some VMs.

Submodule libvfio-user a646db0b7..5af77514b:
  > bump MAX_DMA_REGIONS (#800)

Signed-off-by: John Levon <john.levon@nutanix.com>
Change-Id: I12eb09c1e4ab3f75082c4c86533eea42f7f2cf47
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/23566


Reviewed-by: Jim Harris <jim.harris@samsung.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>