hw/i386/pc: Remove vmport value assertion

hw/i386/pc: Remove vmport value assertion

There is no need for this assertion here, as we only use vmport value
for equality/inequality checks. This was originally prompted by the
following Coverity report:
 >>> CID 1559533:  Integer handling issues (CONSTANT_EXPRESSION_RESULT)
 >>> "pcms->vmport >= 0" is always true regardless of the values of
 >>> its operands. This occurs as the logical first operand of "&&".

Signed-off-by: Kamil Szczęk <kamil@szczek.dev>
Reported-By: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/ZwF9ZexNs1h-uC0MrbkgGtMtdyLinROjVSmMNVzNftjGVWgOiuzdD1dSXEtzNH7OHbBFY6GVDYVFIDBgc3lhGqCOb7kaNZolSBkVyl3rNr4=@szczek.dev
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>