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 <nikita.leshchenko@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200312165431.82118-9-liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>