bhyve: Describe COM3 and COM4 in the DSDT only if they are configured

[wanpengqian_gmail.com]

Summary

bhyve always emulates the four LPC UARTs and always describes them in
the DSDT with their legacy resources: COM1 (0x3f8) and COM3 (0x3e8)
both with IRQNoFlags {4}, COM2 (0x2f8) and COM4 (0x2e8) both with {3}.
IRQNoFlags descriptors are exclusive and there is no _PRS, so Windows
finds two devices claiming the same IRQ and starts none of them: all
four ports show code 12 ("This device cannot find enough free resources
that it can use") in Device Manager.

Describe COM1 and COM2 as before and COM3 and COM4 only if they are
configured with -l com3,... or -l com4,... The UARTs themselves are
still emulated, so the I/O ports and the snapshot layout do not change.
Two configured ports on one IRQ (com1 and com3) still conflict in
Windows, as they would on real hardware.

Signed-off-by: Wanpeng Qian <[e-mail]>
Sponsored by: keelos.dev

Test Plan

Built usr.sbin/bhyve on main 04a3a6e3a with and without the change and
booted the same FreeBSD 16.0-CURRENT guest (bhyveload, -A, -s 31,lpc
-l com1,tcp=...) with each.

Without the change the DSDT has COM1-COM4 and the guest attaches four
UARTs, two pairs on one IRQ each:

uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0
uart2: <16550 or compatible> port 0x3e8-0x3ef irq 4 on acpi0
uart3: <16550 or compatible> port 0x2e8-0x2ef irq 3 on acpi0

With the change the DSDT has COM1 and COM2 only, and the guest attaches
uart0 and uart1 as before. With -l com3,... COM3 is described as well.

Windows 10 guest (UEFI, -A): without the change all four COM ports show
code 12 in Device Manager; with the change COM1 and COM2 work. (This
part was tested with the same change on 14.5's bhyve, which has the
same pci_lpc.c code.)

Compile-tested on fbsd-head (main 083b44ddc + this commit): usr.sbin/bhyve with -Werror, 0 warnings.