OvmfPkg: XenPvBlkDxe: handle empty cdrom drives

OvmfPkg: XenPvBlkDxe: handle empty cdrom drives

Empty cdroms are not going to connect, avoid waiting for the backend to
switch to state 4, which is never going to happen, and return
error instead from XenPvBlockFrontInitialization(). Detect an
empty cdrom by looking at the "params" node on xenstore, which is set to
"" or "aio:" for empty drives by libxl.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18651 6f19259b-4bc3-4df7-8a09-765794883524