fix: add 640th KiB to the available memory

As described in the issue #3044 FC was only announcing 639 KiB
as the first memory region available to the guest.

This commit fixes it by replacing `EBDA_START` value with `EBDA_END`
`EBDA_END` marks the end of the EBDA region and first memory region (640 KiB)

Signed-off-by: Egor Lazarchuk <[email protected]>

src/vmm/src/arch/x86_64/mod.rs

@@ -41,8 +41,10 @@ pub enum ConfigurationError {
     InitrdAddress,
 }
 
-// Where BIOS/VGA magic would live on a real PC.
-const EBDA_START: u64 = 0x9fc00;
+// EBDA is located in the last 1 KiB from 0x9FC00 - 0x9FFFF
+// This values is used to mark first 0x0 - EBDA_END region
+// of usable memory to the guest.
+const EBDA_END: u64 = 0xa0000;
 const FIRST_ADDR_PAST_32BITS: u64 = 1 << 32;
 /// Size of MMIO gap at top of 32-bit address space.
 pub const MEM_32BIT_GAP_SIZE: u64 = 768 << 20;
@@ -134,7 +136,7 @@ pub fn configure_system(
         params.hdr.ramdisk_size = initrd_config.size as u32;
     }
 
-    add_e820_entry(&mut params, 0, EBDA_START, E820_RAM)?;
+    add_e820_entry(&mut params, 0, EBDA_END, E820_RAM)?;
 
     let last_addr = guest_mem.last_addr();
     if last_addr < end_32bit_gap_start {