megasas: fix guest-triggered memory leak

If the guest sets the sglist size to a value >=2GB, megasas_handle_dcmd
will return MFI_STAT_MEMORY_NOT_AVAILABLE without freeing the memory.
Avoid this by returning only the status from map_dcmd, and loading
cmd->iov_size in the caller.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
