vmware_vga: Check cursor dimensions passed from guest to avoid buffer overflow

Check that the cursor dimensions passed from the guest for the
DEFINE_CURSOR command don't overflow the available space in the
cursor.image[] or cursor.mask[] arrays before copying data from the
guest into those arrays.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>