Increase nr_uart to 4

crosvm always (regardless of how many --serial flags are used) adds four
UART devices.

Bug: 236379922
Test: watch TH
Change-Id: Iedc4da510f7330ac4578aac81781d03f1ad8d5cd
diff --git a/microdroid/bootconfig.common b/microdroid/bootconfig.common
index 362ff23..f2c8788 100644
--- a/microdroid/bootconfig.common
+++ b/microdroid/bootconfig.common
@@ -1,3 +1,3 @@
 androidboot.first_stage_console = 1
 androidboot.hardware = microdroid
-kernel.8250.nr_uarts = 2
+kernel.8250.nr_uarts = 4