xm: Fix hot-unplug of statically-assigned devices

xm: Fix hot-unplug of statically-assigned devices

Prior to changset 19510:5c69f98c348e - 'xm, xend: Replace "vslt" with
"vslot"', both vslt and vslot were used in the xm code, often fairly
arbitrarily.

However, in the dictionary that describes a pci function both vslt and
vslot were present. vslt stored the slot assigned to the function. And
vslot stored the slot the user requested for the function, or
AUTO_PHP_SLOT if no slot was requested.

With the renaming these two values got merged into a single entry.
This patch un-merges them by renaming the what was vslot to
requested_vslot.

So an out of chronological order list of name changes is:

'vslot' -> 'requested_vslot'
'vslt'  -> 'vslot'

Signed-off-by: Simon Horman <horms@verge.net.au>