hw/i386/vmport: Define enum for all commands

hw/i386/vmport: Define enum for all commands

No functional change.

Defining an enum for all VMPort commands have the following advantages:
* It gets rid of the error-prone requirement to update VMPORT_ENTRIES
when new VMPort commands are added to QEMU.
* It makes it clear to know by looking at one place at the source, what
are all the VMPort commands supported by QEMU.

Reviewed-by: Nikita Leshenko <[email protected]>
Signed-off-by: Liran Alon <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20200312165431[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>