Add new string testing functions which return a y/n result:
 eq : are two strings equal (ignoring leading/trailing space)?
 ne : are two strings unequal?
 isempty : is a string empty?
 notempty : is a string non-empty?

Based on an idea by Ákos Kovács <akoskovacs@gmx.com>.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>