Show length mismatch error is hex

Show length mismatch error is hex

When live migrate fails due to a section length mismatch we currently
see an error message like:

Length mismatch: 0000:00:03.0/virtio-net-pci.rom: 10000 in != 20000

The section lengths are in fact in hex, so this should read

Length mismatch: 0000:00:03.0/virtio-net-pci.rom: 0x10000 in != 0x20000

Correct the error string to reflect this.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>