OvmfPkg: VirtioNetDxe: fix some build errors emitted by Visual Studio

OvmfPkg: VirtioNetDxe: fix some build errors emitted by Visual Studio

These were found with the gcc-4.4 option "-Wconversion" after Jordan
reported the build failure under Visual Studio. The patch was originally
posted to edk2-devel as "silence.patch":

http://thread.gmane.org/gmane.comp.bios.tianocore.devel/2804/focus=2972

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14419 6f19259b-4bc3-4df7-8a09-765794883524