From the Virtualbox website: "Please note that the original 4.3.10 packages (revision 92957) were replaced (revision 93012) due to a last-minute regression, see #12868!" consequently the virtualbox-extpack-oracle-4.3.10 ebuild has a wrong filename in SRC_URI (i.e. wrong MY_BUILD)
New path: http://download.virtualbox.org/virtualbox/4.3.10/VirtualBox-4.3.10-93012-Linux_amd64.run
+*virtualbox-extpack-oracle-4.3.10.93012 (27 Mar 2014) + + 27 Mar 2014; Lars Wendler <polynomial-c@gentoo.org> + -virtualbox-extpack-oracle-4.3.10.ebuild, + +virtualbox-extpack-oracle-4.3.10.93012.ebuild: + Upstream re-released 4.3.10 version. +