configure: Ignore --erratatxt when --target=kvmtool

kvmtool runs a test using the -f/--firmware argument, which doesn't load an
initrd, making specifying an errata file useless. Instead, configure forces
all erratas to be enabled via the CONFIG_ERRATA_FORCE define in
lib/config.h.

Forbid the --erratatxt option when kvm-unit-tests is configured for kvmtool
and let the user know that all erratas are enabled by default.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>