tools: add option to explicitly enable VirtFS in QEMU build

tools: add option to explicitly enable VirtFS in QEMU build

9pfs support has been a documented feature since Xen 4.9, but QEMU will
not be built with backend support unless VirtFS is enabled, which is
predicated on the libcap and libattr dev packages being installed. This is
not obvious to anyone intending to use 9pfs.

This patch adds an 'enable-9pfs' option to configure which, if specified,
will cause '--enable-virtfs' to be passed to QEMU's configure. This will
cause the dependency on libcap and libattr to be called out if the packages
are not in installed.

For completeness, specifying 'disable-9pfs' will cause '--disable-virtfs' to
be passed to QEMU's confgure and not specifying an option will keep the
previous behaviour of predicating VirtFS on whether the libcap and libattr
packages are installed.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>