spapr: Provide an error message when migration fails due to htab_shift mismatch

spapr: Provide an error message when migration fails due to htab_shift mismatch

Include an error message when migration fails due to mismatch in
htab_shift values at source and target. This should provide a bit more
verbose message in addition to the current migration failure message
that reads like:

qemu-system-ppc64: error while loading state for instance 0x0 of device 'spapr/htab'

After this patch, the failure message will look like this:

qemu-system-ppc64: htab_shift mismatch: source 29 target 24
qemu-system-ppc64: error while loading state for instance 0x0 of device 'spapr/htab'

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>