Based on http://www.mail-archive.com/qemu-commits@nongnu.org/msg00202/qemu.x86_boot_vidmode.patch. 

diff -Nupr qemu/hw/pc.c qemu-new/hw/pc.c
--- qemu/hw/pc.c	2009-05-26 16:10:44.000000000 +0800
+++ qemu-new/hw/pc.c	2009-05-26 17:17:25.000000000 +0800
@@ -593,6 +593,78 @@ static long get_file_size(FILE *f)
     return size;
 }
 
+static uint16_t hato16i(char *hex)
+{
+	uint16_t integer = 0;
+	if(hex[1] == 'x')
+	   hex += 2;
+
+    while(*hex != '\0'){
+		integer = (integer<<4);
+
+		if(*hex >= 0x30 && *hex <= 0x39){ /*0~9*/
+			integer += *hex - 0x30;
+		}else if(*hex >= 0x61 && *hex <= 0x66){ /*a~f*/
+			integer += *hex - 0x61 + 0xa;
+		}else if(*hex >= 0x41 && *hex <= 0x46){ /*A~F*/
+			integer += *hex - 0x41 + 0x1;
+		}else
+			break;
+
+		hex++;
+	}	
+	return integer;
+}
+
+static uint16_t ato16i(char *dec)
+{
+	uint16_t integer = 0;
+
+    while(*dec != '\0'){
+		integer *= 10;
+
+		if(*dec >= 0x30 && *dec <= 0x39){ /*0~9*/
+			integer += *dec - 0x30;
+		}else
+			break;
+
+		dec++;
+	}	
+	return integer;
+}
+
+static int get_vga_mode(const char* kernel_cmdline, uint16_t *vga_mode)
+{
+	char mode[7];
+	int i = 0;
+	char *p	= strstr(kernel_cmdline, "vga=");
+	if(p == NULL)
+		return 1;
+
+	p += 4;
+
+	while(*p != ' ' && *p != '\0' && i < 6)	{
+		mode[i] = *p;
+		i++;
+		p++;	  
+	}
+	mode[i] = 0;
+
+	if(!strncmp(mode, "ask", 3)){
+		*vga_mode = 0xfffd;
+	}else if(!strncmp(mode, "normal",6)){
+		*vga_mode = 0xffff;
+	}else if(!strncmp(mode, "ext", 3)){
+		*vga_mode = 0xfffe;
+	}else if(!strncmp(mode, "0x", 2)){//in hexdecimal 
+		*vga_mode = hato16i(mode);
+	}else	//in decimal 
+		*vga_mode = ato16i(mode);
+
+	return 0;
+}
+
+ 
 static void load_linux(target_phys_addr_t option_rom,
                        const char *kernel_filename,
 		       const char *initrd_filename,
@@ -605,6 +676,7 @@ static void load_linux(target_phys_addr_
     uint16_t real_seg;
     int setup_size, kernel_size, initrd_size, cmdline_size;
     uint32_t initrd_max;
+	uint16_t vid_mode;
     uint8_t header[1024];
     target_phys_addr_t real_addr, prot_addr, cmdline_addr, initrd_addr;
     FILE *f, *fi;
@@ -683,6 +755,12 @@ static void load_linux(target_phys_addr_
     if (protocol >= 0x200)
 	header[0x210] = 0xB0;
 
+	/*parse cmdline and set vga mode*/
+	if(!get_vga_mode(kernel_cmdline, &vid_mode)){
+		header[0x1fa] = vid_mode&0xff;
+		header[0x1fb] = (vid_mode&0xff00)>>8;
+	}
+
     /* heap */
     if (protocol >= 0x201) {
 	header[0x211] |= 0x80;	/* CAN_USE_HEAP */