vc: use a common prefix for chr callbacks

vc_chr_write() is more appropriate than _puts() since no newline is
appended, even though it's not used only as a callback.

Keep "qemu_chr_parse" prefix, most chardev parse functions use this
prefix atm.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>