hyperv storvsc: Don't abuse struct sglist to hold virtual addresses.

[jhb]

Summary

struct sglist is intended for holding S/G lists of physical address
ranges, not virtual address ranges. GCC 9.x issues several warnings
due to casts between pointers and integers of different sizes as a
result (vm_paddr_t is 64-bits on i386). Instead, add a local 'struct
hv_sglist' which uses an array of 'struct iovec' to hold the S/G list
of virtual address ranges.

[emaste]

This looks reasonable to me but I will defer to @whu for detailed review.

[whu]

Please give me a few days to review. I am tied up with other issues right now. Thanks!