staging: unisys: virtpci: Allocate memory using the size of the variable

Change two instances of using the sizeof a structure to the size of a specific variable when
allocating memory in virtpci.c

Signed-off-by: Bryan Thompson <bryan.thompson@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>