Refactor code to trim file extension

In some parts of the code, it is necessary to remove the file extension
from a string. Currently, the last 5 characters are removed from
the text.

Remove the file extension by searching for the last '.' inside the
string. This approach works for other file extension lengths - even
though we will probably use only .html - and is more clear than having
a magic number ("5") inside the code.
Also, add a comment explaining what the substring method does in
that case.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
Define the default tab more clearly

In order to define the default tab, the developer needs to update the
code in more than one place. Also, the corresponding variable name does
not reflect what it actually means.

Define the default tab in a single variable and make all the other
values dependent on it.
Also, rename the variable from "firstTab" to "defaultTab".

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
Change the default tab to "Guests"

When Kimchi is launched, it opens the tab "Host" by default. As Kimchi
is a virtual machine manager, it makes more sense to use the default tab
"Guests".

Update the default tab from "Host" to "Guests".

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>