Fix default feature breakage

A recent change
(1d3a8af)
made it so some previous code using `SerialPort::new()` wouldn't
compile if the user was using the default feature set.

Fixing this to just be `default = [ "nightly" ]` sets the same features
for `x86_64`, while preventing breakage.

See: cloud-hypervisor/rust-hypervisor-firmware#66

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

Cargo.toml

@@ -12,7 +12,7 @@ bitflags = "1.1.0"
 x86_64 = { version = "0.12.2", default-features = false, features = ["instructions"] }
 
 [features]
-default = [ "x86_64/default" ]
+default = [ "nightly" ]
 stable = [ "x86_64/external_asm" ]
 nightly = [ "x86_64/nightly" ]