Tools: hv: update buffer handling in hv_fcopy_daemon

Currently this warning is triggered when compiling hv_fcopy_daemon:

hv_fcopy_daemon.c:216:4: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
    kernel_modver = *(__u32 *)buffer;

Convert the send/receive buffer to a union and pass individual members as
needed. This also gives the correct size for the buffer.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>