qemu: fix host virtfs option

qemu: fix host virtfs option

Fix the build of host-qemu with virtfs enabled: fix a typo in makefile
conditional and add a dependency on host-libcap as that is a dependency of
virtfs support:

    if test "$virtfs" != no && test "$cap" = yes && test "$attr" = yes ; then
      virtfs=yes

The virtfs configuration option was added by commit e0f49e648401
("package/qemu: add option to enable virtual filesystem in host qemu").

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>