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

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.

Differential Revision: https://reviews.freebsd.org/D31933