emulators/qemu-utils: Don't depend on GTK3 or cairo.

r499318 switched qemu from GTK2 to GTK3, accidentally making GTK3 a
dependency of qemu-utils. This commit fixes that, and also removes cairo as
a dependency for qemu-utils.

PR: 237738
Reported by: bsam
Reviewed by: bsam, bofh
Approved by: bofh (qemu maintainer)
Differential Revision: https://reviews.freebsd.org/D20202