Make qemu_log_mask() a macro which only calls the function to
do the actual work if the logging is enabled. This avoids making
a function call in possible fast paths where logging is disabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>