OvmfPkg/Sec: Add FindFfsSectionInstance

OvmfPkg/Sec: Add FindFfsSectionInstance

This allow you to search for an 'instance' of a section
within a series of FFS sections.

For example, we will split the MAINFV into a PEI and DXE
FV, and then compress those two FV's together within a
FFS FV file. The DXE FV will appear as the second section
of the file, and therefore we will search for it using
an Instance=1 value.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15150 6f19259b-4bc3-4df7-8a09-765794883524