OvmfPkg: extract VirtioLib from VirtioBlkDxe

OvmfPkg: extract VirtioLib from VirtioBlkDxe

Introduce a new library called VirtioLib, for now only collecting the
following reusable functions with as little changes as possible:

- VirtioWrite()
- VirtioRead()
- VirtioRingInit()
- VirtioRingUninit()
- AppendDesc()

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13842 6f19259b-4bc3-4df7-8a09-765794883524