[build] Add named configuration for qemu

Add a named configuration for qemu, based on the config.ipxe.general.h
file taken from the current qemu repository and enabling the option to
work around the missing EFI_PXE_BASE_CODE_PROTOCOL.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/qemu/general.h

+/* Disable entry during POST */
+#undef ROM_BANNER_TIMEOUT
+#define ROM_BANNER_TIMEOUT 0
+
+/* Extend banner timeout */
+#undef BANNER_TIMEOUT
+#define BANNER_TIMEOUT 30
+
+/* Work around missing EFI_PXE_BASE_CODE_PROTOCOL */
+#define EFI_DOWNGRADE_UX