Update 'info migrate' to display migration blocking information.
If the outbound migration is not blocked, there is no change, however
if it is blocked a message is displayed with a list of reasons why,
e.g.

qemu-system-x86_64 -nographic  -smp 4 -m 4G -M pc,usb=on \
 -chardev null,id=n -device usb-serial,chardev=n \
 -virtfs local,path=/home,mount_tag=fs,security_model=none \
 -drive if=virtio,file=myimage.qcow2

(qemu) info migrate
globals:
store-global-state: on
only-migratable: off
send-configuration: on
send-section-footer: on
decompress-error-check: on
clear-bitmap-shift: 18
Outgoing migration blocked:
  Migration is disabled when VirtFS export path '/home' is mounted in the guest using mount_tag 'fs'
  non-migratable device: 0000:00:01.2/1/usb-serial

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210202135522.127380-3-dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>