tests/acceptance: look for target architecture in test tags first

tests/acceptance: look for target architecture in test tags first

A test can, optionally, be tagged for one or many architectures.  If a
test has been tagged for a single architecture, there's a high chance
that the test won't run on other architectures.  This changes the
default order of choosing a default target architecture to use based
on the 'arch' tag value first.

The precedence order is for choosing a QEMU binary to use for a test
is now:

 * qemu_bin parameter
 * arch parameter
 * arch tag value (for example, x86_64 if ":avocado: tags=arch:x86_64
   is used)

This means that if one runs:

 $ avocado run -p qemu_bin=/usr/bin/qemu-system-x86_64 test.py

No arch parameter or tag will influence the selection of the QEMU
target binary.  If one runs:

 $ avocado run -p arch=ppc64 test.py

The target binary selection mechanism will attempt to find a binary
such as "ppc64-softmmu/qemu-system-ppc64".  And finally, if one runs
a test that is tagged (in its docstring) with "arch:aarch64":

 $ avocado run aarch64.py

The target binary selection mechanism will attempt to find a binary
such as "aarch64-softmmu/qemu-system-aarch64".

At this time, no provision is made to cancel the execution of tests if
the arch parameter given (manually) does not match the test "arch"
tag, but it may be a useful default behavior to be added in the
future.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-7-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>