portageq: add new expand_virtual function

portageq: add new expand_virtual function

Something like this was requested in bug #157357. Now that Gentoo has
migrated all virtuals to GLEP 37 new-style virtuals, this kind of
function may be helpful in order to resolve the currently installed
provider of a particular virtual in scripts like bootstrap.sh.

Usage:

	portageq expand_virtual <root> <atom>

		Returns a \n separated list of atoms expanded from a
		given virtual atom, excluding blocker atoms. Satisfied
		virtual atoms are not included in the output, since
		they are expanded to real atoms which are displayed.
		Unsatisfied virtual atoms are displayed without
		any expansion. The "match" command can be used to
		resolve the returned atoms to specific installed
		packages.

Example input/output:

	$ portageq expand_virtual / virtual/jre
	=dev-java/sun-jdk-1.6.0*

	$ portageq expand_virtual / virtual/jre:1.5
	dev-java/gcj-jdk

	$ portageq expand_virtual / virtual/package-manager
	sys-apps/portage

	$ portageq expand_virtual / virtual/libc
	sys-libs/glibc:2.2

	$ portageq expand_virtual / virtual/os-headers
	sys-kernel/linux-headers:0