[HVM] Allow more dynamic use of the qemu serial lines by allowing the Summagraphics...

[HVM] Allow more dynamic use of the qemu serial lines by allowing the Summagraphics port to float.

qemu will emulate up to 4 serial lines (COM1 - COM4).  One of these
lines supports a Summagraphics protocol.  This has changed recently,
and it's now always COM2.  This patch allows the user to specify
to qemu which lines should be serial lines, and the code will place
the Summagraphics compatible line at just above the last specified
serial line.  qemu defaults serial line 0 (COM1) to be "vc", resulting
in the Summagraphics support residing on COM2. This is in keeping
with the current behavior.

The results, via qemu command line, are:
<none>                                    COM2
-serial x                                 COM2
-serial x -serial x                       COM3
-serial x -serial x -serial x             COM4
-serial x -serial x -serial x -serial x  <none>

The results are deterministic, and allow the use of up to 4 serial
lines including none with Summagraphics support.  There is no
change from current behavior, unless requested.

Signed-off-by: Ben Thomas <ben@virtualiron.com>