hv: vuart: fix the data type violations

- Fix the data type violations based on MISRA-C requirements
- Add '-fsigned-char' in Makefile to let the compiler treats 'char' be
   signed, like 'signed char'.
  Otherwise, the static checker treats 'char', 'signed char' and 'unsigned
   char' as three different types.
- Fix some minor coding style issues, such as TAB issues, line over 80
   characters issues and comments style

v1 -> v2:
 * fix the violation regarding to 'fifo_getchar'

Tracked-On: #861
Signed-off-by: Shiqing Gao <shiqing.gao@intel.com>
Reviewed-by: Junjie Mao <junjie.mao@intel.com>

hypervisor/Makefile

@@ -37,6 +37,7 @@ include scripts/kconfig/kconfig.mk
 CFLAGS += -Wall -W
 CFLAGS += -ffunction-sections -fdata-sections
 CFLAGS += -fshort-wchar -ffreestanding
+CFLAGS += -fsigned-char
 CFLAGS += -m64 -mno-mmx -mno-sse -mno-sse2 -mno-80387 -mno-fp-ret-in-387
 CFLAGS += -mno-red-zone
 CFLAGS += -nostdinc -nostdlib -fno-common