OvmfPkg: update qemu-executable for IA32

Previously for IA32, we would only try to run qemu. Newer releases
of QEMU now have renamed the x86 qemu to qemu-system-i386.

Now, we search for:
1. qemu-system-i386
2. qemu-system-x86_64
3. qemu

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14101 6f19259b-4bc3-4df7-8a09-765794883524