OvmgPkg/PlatformBootManagerLib: Add Debug Agent console

OvmgPkg/PlatformBootManagerLib: Add Debug Agent console

The Debug Agent in the SourceLevelDebugPkg can multiplex
both source level debug messages and console messages on
the same UART.  When this is done, the Debug Agent owns
the UART device and an additional device handle with a
Serial I/O Protocol is produced with a VenHw device path
node.

In order for a platform to provide a UART based console
when the Debug Agent is using the same UART device, the
PlatformBootManagerLib must consider the SerialI/O
Protocol produces by the Debug Agent as one of the
supported consoles.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>